So, I've spent about six months of my life meditating, never reached the first Jhana. Fast forward to now, and I've been practicing consistently for over a month. Yesterday I think I experienced the first jhana, but I'm not sure.

I was about fifteen minutes into my meditation with strong access concentration, focusing on a pleasant sensation near my heart. The concentration became passive for a few moments, and next thing I knew the meditation felt very different. That feeling I had in my chest filled my whole head, and the best way I can describe it would be to say it was the exact opposite of a headache. Felt very nice. Also, sounds seemed farther away and muffled, as if I was hearing them underwater. My concentration was a little stronger, but not much. When I decided I'd had enough, it took about 30 seconds with my eyes open to fully get rid of that far-away feeling. 
I've meditated a few more times since then, and I guess I'm trying too hard again. I can start building up the feeling and sounds will get muffled, but have yet to reproduce a full immersion like the first time.

Was this the first jhana? If not, what the heck was it?

Rebecca, the major block to jhana/fruition/attainments are the desire to attain them itself. You may have or may not have reached it - that does not matter. What matters is that you have steady access to it, isn't it? I will practice to be able to enter, exit and re-enter it at will before lingering longer and working towards the 2nd. The Buddha warned rightly of this. I would work at attaining Access Concentration with mastery before the 1st Jhana. What you have described where you start to get immersed, i.e. sensing the beginning of an effortless concentration is the entry. The other chest/head symptoms change with time/other factors, just notice the out-of-the-ordinary stuff, keep a safe distance (no crave/cling) and enjoy them. Going slow (enough for comprehension) and steady will prevent unnecessary problems and suffering...

The classic reply is: Keep Going. The answer lies within further practice.

All the best!

Hello again, Yilun Ong!

I know from reading MCTB that I'm nowhere near close to being ready to enter the second jhana. Hopefully it's not something that can happen by accident. I'm probably giving myself too much credit. 
For now I'm going to try to ease up a bit, not force it, and if first jhana is gonna happen again, if at all, it happens. I can't help but be curious about my experiences though. 

Hi Rebecca, did you notice a change in state, e.g. an almost total change in scene (see/feel/hear) after a semi-complete interruption to senses with an intermission/transition like a white or blackout in visual sense? IME, that is what happens during jhana-entry and transitions. If not, what happened is that you were in AC and raptures (5 grades of them) were progressing along. You have managed to move attention to raptures without breaking concentration, just keep going and keep a lid on excitement and desire. Be happy too!

It absolutely can happen by accident (and that's not a bad thing). There's a great quote something along the lines of "Enlightenment is an accident, but you can make yourself accident prone". As Yilun suggested, the answer is pretty much always "just keep practising" (and don't make too many assumptions about what you are or are not ready for).

It is important to realize that various authors and teachers and pundits span a wide range of how they define “first jhana”, and consequently, what you did, which is describe phenomenology, what you felt, how long it lasted, what came before it, what came after it, is vastly more valuable than what typically happens in the dharma world, which is that people argue about the standards for the jhana in question.

In other words, if the experience increases concentration, increases motivation, suppresses hindrances, provides faith, and furthers practice, then it has value, regardless of what people using widely varying criteria call it.

The world of jhana classification is pretty neurotic at this point in the contemporary world. There is this vast range of experiences that might be classified as some sort of jhana, yet we only have eight terms from the Sanctified Past to describe this vast range. It would be as if you described a location on the earth using only 7 continents and the ocean and wouldn’t be more specific than that.

Clearly, you are getting into something that has jhanic factors, and some people would definitely call it first jhana (such as Ayya Khema, Leigh Brasington, etc.) and others definitely wouldn’t (BAWallace, some of the Thai and Thai-influenced Ajahns), but regardless, that’s the right direction to go in if you are working on samatha practice.

I actually map things jhana wise much more in shades of grey and in terms of degrees, liking phenomenology like what you described a lot better than people who just say “I was in the first jhana” without qualification, given the huge range of what people mean by the term. Unless you know the tradition someone is coming from and how it defines jhana, it can be nearly impossible to know what they mean by that, so thanks for your descriptions and best wishes in your practice.

Yilun Ong,
It's almost like I fell asleep for a second, and when I "woke up" I was in this new state of being. Granted I'm pretty sure it wasn't sleep. I wasn't feeling tired and I had remained sitting upright for the duration, but it was as if consciousness took a momentary break. 

Daniel M Ingram, 
I would agree that whatever it was that I experienced, it had a positive effect on my practice. I'm happy about that. 

Thanks to all of you who weighed in. Plenty for me to think about!

Rebecca, in short, just do NOT suffer wanting to repeat the experience (and all meditative experiences)! Everyone has plenty "What the f^@# was that?" experiences, and I encourage you to keep sharing them as I think a 'Sangha' helps...

Now what you described could even be a cessation (Fruition) event but dharma diagnostics is very tricky, so the before/after/repeatability is much more useful than a singular occurence. I'd drop this possibility and keep practicing.

Constructive thoughts (reviewing) would be revolving around what good things you were building (+bad stuff you let go) to allow for such good progress. These details are important (you can hone your mindfulness by noticing them and being able to describe them thereafter - never strive to remember things during meditation) but do not obsess as well. There's plenty of uncomfortable stuff (and the good stuff) coming your way, just do not add unnecessary suffering nor avoid these discomforts. 

I am sure you are doing just fine! 

Edit: I would start a practice log, talk about myself, my long/short-term goals, my problems (I want to be honest, open and expressive - enjoy the joys of that online before expanding). I will be detailed so others can help me better and that drives mindfulness during practice. I will limit questions in it and ask them in separate topics. I will use it as a commitment to my practice and enjoy the process. The information I provide will benefit others as well...

You know I was just the other day thinking "hey, these people on the forum are pretty much my Sangha!" I'm happy about that!

I was thinking about starting a practice log, but didn't want to embarass myself with this beginner's mind I have. Since you suggested it, I think I'll go ahead and give it a go. That way people know what I'm working on, where my weaknesses are, and hopefully, how I can be helped to make more progress.

I've dropped the striving for that return to what might have been the first jhana. Concentration practice as usual. It will come along in time.