Since last year, around this time, my health has been declining or something.

I can't eat as much in one sitting anymore.
When I eat a lot, I feel like I want to throw up, which has usually never happened before last year.
If I drink more than two glasses (more than 5 or 6 fl. ounces) of water, I feel really sick and have to lay down.
I can no longer sprint for more than 5 seconds.
I have the sneaking suspicion that it's just a lot of gas.
I've had trouble with bowel movements; details too embarrassing and probably inappropriate to post here.

I think this might have something to do with the fact that I can't burp, but I don't know. I've never been able to burp before, so I don't know why it might be affecting me now.

I want to go back to being able to down a whole Gatorade after a good day's workout, but I don't know what might be wrong with me.
I think I might have a tapeworm or something(?) since I've never been able to gain much weight after these symptoms started showing.
I figured it was just something temporary that would go away with some more cross country training, but I don't think so anymore, now that it's been a whole year.

EDIT: Forgot to mention, whenever I do feel sick like this, I build up a whole lot of phlegm in my throat.

Anyone know what could be wrong with me?