My little brother has been a mess for the last week. He had a "stomach flu" on Thursday with vomiting and diarrhea, had diarrhea/soft poop friday, but went like 6 times that day, and today he keeps crying and saying that his butt hurts, yet he won't poop. He's 3 years old and has been taking some kind of medicine for a sinus infection. He's done with it now, but could that be the culprit?