Why do i get bad gas, and abdominal pain sometimes after i eat: one medium size slice of lou malnati's deep dish custom made pizza? (i am underweight 5'8, and 110 pounds. struggling to gain weight since february when suddenly started to lose weight?

Assuming lots of cheese, you might be lactose intolerant. Or gluten intolerant, assuming a wheat-based crust. If so, other dairy or gluten-containing foods would cause the same symptoms. Other intestinal problems could do the same, such as small intestinal bacterial overgrowth (SIBO). Any of these could explain underweight. See your doctor -- or preferably a gastroenterologist. Good luck.