Can aquarium fish throw up thier full stomach or is it a parasite that's been livng in them for a long time?

I am really confused. My MALE guppy has been bloated since two days ago when i fed him a pea. Normally his stomach would go down by the next day if he eats alot (which he will if given the chance) but since then his stomach has been bloated. i thought he might have caught what my other fish had and so i put some maraycn two by mardel in his tank to see if that would help. immediatly afterwards he regurgitated a long white string out of his MOUTH! It was really scary and before this he did not show any signs of being sick nor does he now while being sick. he will still eat, fight with the other fish time to time (nonaggressvely) and will peck at the gravel. I havent fed him for the past two days unless my brother did. What could be wrong with him.