First thing's first.....This doesn't sound like you're just doing anapanasati, what you've said suggests that you're paying attention to sensation in your body, in which case you're actually doing vipassana. If that is what you're actually doing then my interpretation of your post, speculation and opinion as always, would be as follows:
You have strong concentration already developed which makes access concentration [1] easy, you've managed to note your way to the 2nd ñana, Cause & Effect, one of the signs of this stage is this mechanical and jerky breathing when attention is placed on the breath. Well done, you've started doing insight practice!
When I slow down the breath I can feel as if I'm going "out of it" and so I try to just let the breath be natural again but it gets very uncomfortable, shallow, fast etc.
Cause. Effect. Simple.
The pleasure feels like liquid is being pumped out of some organ in upper abdomen and right at the source of pleasure, in the abdomen there is a moderate pain
Well done for paying attention to the physical location of these sensations, that sounds as near as damn it to noting as you're likely to get. Note "pumping", note "pleasure", note "pain". Just name each thing you become aware of in that moment, you're not solidifying these sensations to a jhana, you're seeing them arise and pass. When you "un-concentrate", note it as whatever it is, e.g. "distracted" or whatever sensation you're aware of.
I've already tried to just go with it, that's really hard, like... I feel like I'm going to die after about 5 minutes
Welcome to the 3rd ñana, The Three Characteristics, prepare for possible discomfort, pain, tension, frustration, crying and facing up to reality in it's most transient, unsatisfying and self-less form so far.....and it gets worse when you hit Dark Night. I really don't want to seem like I'm discouraging you but it's better that you know that this whole insight thing isn't all sweetness and light, there's a lot of heavy shit to face up to but it's all worthwhile in the long run.
You need to be willing to accept that you're going to go through some really difficult times if you continue down this path. If you're meditating the way you say you are then it's only a matter of time before you hit the 4th ñana, the Arising & Passing, which is the point of no return and from then you're "on the ride" until you get stream entry so think carefully about what you're willing to do to attain your goals. After the A&P things can get very, very, very difficult, although there's always the chance that you'll cross Dark Night quickly if you maintain solid, consistent and accurate practice, so you should consider whether you've got your psychological trip together sufficiently enough to handle the forthcoming stages of insight in a skillful manner.
My crazy theory is that whichever organ is pumping my piti is overworked... lol, I guess that would be pancreas or um... liver..? I'm not too good with no fancy biology
Shut up.

Don't get into trying to map this to any sort of biological model (yet), just note the sensations and you'll get along fine. It's so simple that you'll risk making it stupidly complicated while not getting any fundamental insight! The same goes for psychological models, they're useful to a point but all of these maps and models are just that.....maps and models, remember that the map is not the territory it describes so stick with practice and you'll get a much better understanding than you would by just thinking about it, or reading about it.
maybe this is linked to this weird thing I do where I exhale deeply and slowly from the bottom of my longs inducing a strong, localized, momentary pleasure, anyone else get that?
It's not weird although I totally understand your confusion, this is all a totally natural process available to anyone with the ability to apply the techniques. What you describe here sounds, to me at least, like momentary concentration on the sensations at the end of the exhalation. It's noting that stop, the pause between each inhale and exhale when you're understanding of what you once saw as a continuous phenomenon, the breath itself, can be seen to be just as transient and empty as any other sensation. Stay with the sensations of the rising, the pausing, the falling, the pausing, and see breath as a series of sensations.
Just take it easy and don't burn yourself out, practice concentration too and avoid noting sometimes so that you can maintain a good balance. Do what feels right but there's no race for the prize here, the answer is in the journey so take your time and have fun with it.