Sounds like stream-entry to me, but at the same time it could have been an intense A&P. Either way, it's not possible to make an accurate diagnosis based on what you've provided for the simple reason that we all use different language to describe our experience of these things. The limitations of language is a pain in the arse as the words we use to describe this stuff is nothing like the experience itself, but there are certain similarities which become evident with practice. A real-time practice thread would show what's happening in your meditations which is a much better indicator of where you are than a speculative story written after the fact. I know. I did it myself. Check some of my threads and you'll get a laugh.
When I sit right now my mind seems to go into a strong state of concentration, almost jhana-like, on its own within about 15 seconds.
Sounds like you've got stream-entry, although this also depends on what sort of concentration skills you have naturally, or have gained through other meditative practices in the past. Entering an altered state within a few seconds is meant to be attainable by someone at 1st path so that sounds pretty positive to me.
The only way to prove this once and for all is to sit down, meditate and get a fruition. Until you can get a fruition and be absolutely sure, that whole "THAT was it!!" feeling followed by a lovely bliss wave. Start a practice thread, you'll see how beneficial this can be when others who've walked this Path before you and can point out these landmarks. This was how I got confirmation of stream-entry as I was getting regular fruitions in daily life and during the, admittedly scant, meditating I was doing at that time.
I'll be honest with you my meditation sucks. I don't do it nearly often enough. No schedule, no planning, just when I feel like it.
Sounds like me before I got serious about this.
This may be a great hindrance where it comes to progress
Yes. It will. Stop being a lazy bastard.

but I haven't been trying to experience certain things
So what have you been doing? What are you aiming for?
I'm unclear on this stuff, so any and all help at interpreting these experiences is very appreciated.
Listen, I was EXACTLY the same, and I'm still learning, but one of the best ways to learn what's being discussed on here is to get yourself into a meditation routine and examine what's happening by noting sensation as it appears. I didn't get stream-entry through vipassana or any sort of Buddhist techniques, I was a magician (magick, not card tricks...Ha!) with no interest in Buddhism at all, but that changed after stream-entry so I came to this site knowing Jack Shit about this stuff. Now I'm confident enough to be able to give honest advice to people like yourself who are unfamiliar with the basic terminology, but I'm no expert so what I say is just opinion and is subject to correction at any point.
The reason I couldn't give you any "diagnosis" based on what you said is that it's impossible to do so accurately. I could blow smoke up your arse and say I think you're the Buddha but what use would that be to either of us?
Get reading MCTB and ask questions, get into a practice routine and tell us what's happening for you 'cause you'll find that this forum is extremely helpful for learning. Just be with what's going on and don't get too caught up on where you think you might be right now, you'll see soon enough and it's worth the wait for when it all clicks into place.
Metta,
Tommy