Guppy with a HUGE stomach.?

It seems that he has gotten bigger by each day and its only been two days since. He belly seems to be twice the size of him ( may be exageratting a little) but still its really big. Its not pineconeing and he still eats. he also regurgitated a long white string from his mouth but ate it right back agian, Since then it seems that his stomach is getting bigger and bigger and bigger. Someone with experience with fish specifcally guppies please help as i am desperate because ive tried an antibiotic and it hasnt done anything matter of fact it seems to have made him bigger!!!!!!!!!!!!!!!!!!!!!! Oh please help!