This happens to me too. I have searched all over the place looking for a reason. I've gone to the ER twice after having it so bad that I thought my heart was skipping beats and was loosing feeling in my legs and getting light headed. I was also getting muscle twitches, heart palpitations, heart flutters, joint aching, constipation, shortness of breath and anxiety. THEY TOLD ME IT WAS JUST ANXIETY. It seems to be doctors answers for things that they can't see in an xray or blood test. Though after much research I have narrowed it down to a few things.
After I narrowed it down to everything happening after i eat, I began to experiment. I have noticed that I feel much better when I cut out all things that hurt my stomach. I also tried going on a gluten free diet. This actually helped a lot. Also if I eat things like Califlower, turnips, cabbage, brussle sprouts, or rudabegas, I have very bad attacks.
The fact that those things give me palpitations, anxiety, and joint aches makes me think it is due to hypertyroidism, because I have read that those foods can send you into a thyroid storm.
The other option is that it is Ulcers. This can also give you the same symptoms as thyroid, and after a month of not eating wheat and only eating small amounts I do feel a lot better. It is still not totally gone but it is sooo much better. I can actually sleep now.
A wheat alergy is also another option with the same symptoms.
Well I hope this helps. If anyone has any questions just ask.