[yoshi_yoshi]

I only fully read the first paragraph but it sounds like I had a similar experience. I'm 20, skinny, usually exercise, but when it happened, I was in the middle of a very lazy summer when I didn't exercise. I don't drink / smoke.

So for a couple of days in a row during the summer, after dinner, when I went to relax and watch TV, or lie down to sleep, I would notice my whole upper body feeling funny, and that my heart was not beating 'normally'. I would have to stand up and walk around for the feeling to go away. It didn't hurt, but I instinctively knew it was bad. It went on mildly for about three days until the third night at 3am it got really bad. Even when I walked around I was shaking and feeling faint. So I woke up my parents (had never done this before so they weren't annoyed and took me seriously) and they took me to the ER. All the way there my heart rate was really high and I was shaking. I basically had all the things you mentioned in your first paragraph except maybe the impending doom.

They hooked me up to an EKG and surprisingly, the doctor said there was nothing major wrong with me. So they told me just to see my regular doctor. The next day I saw my regular doctor, but he also couldn't tell what was wrong with me, since my tests all came back healthy. Meanwhile, every night I would be experiencing the milder symptoms I described before.

Anyways, this continues for about two weeks, and my doctor still hasn't given me any real help (he had suggested things like not eating full meals, not consuming caffeine). My doctor finally said 'try these pills' and gave me some Prevacid. It worked right away. I took it for about a month, stopped, per his instructions, and haven't had any symptoms since. My doctor seemed puzzled that it worked, since a lot of my symptoms weren't typical of acid-reflux (which Prevacid heals).

I know you said it wasn't a run-of-the-mill acid reflux ordeal, but that one night mine wasn't either. I had the same high heart rate, shivering, and nausea, and for me it would also get a lot worse when I was sitting or lying down. It was weird that it only happened two nights (oops I forgot to mention this happened in isolation one night in the March before the summer, I didn't do anything about it then), every other night it happened was fairly mild and probably a run-of-the-mill acid reflux ordeal. My doctor couldn't really give me an answer on why this was.

Anyways, if this sounds exactly like you, I would recommend going to the doctor of course, but if not, then taking some acid-reflux medication (Prevacid is prescription only, but some others are not) and stop eating full meals. There is a good chance nothing serious is wrong with you. (A high enough chance to stop worrying, but not high enough to not get at least a checkup. It's not that expensive.)