3 year old with rectal pain and abdominal bloating?

My 3 year old daughter complained last Saturday of having pain when peeing. I took her to the doc and they took a urine culture and gave her antibiotics for a probable UTI. I gave the antibiotics for 3 days before the office called and said the culture came back negative. I stopped giving her the medicine on thier orders. Then yesterday she started complaining of pain when she poops. She was sitting on the toilet and crying hysterically about the pain. I was alarmed and we went back the docs again. They examined her and said she was bloated from gas but could see nothing else wrong. She is afraid to poop now. She says its going to hurt. She's not constipated and doesn't have a history of it either. The UTI symptoms resloved on thier own. I am taking her to a pediatric gastro doc on Monday to see what they think. I am so mystified and concerned. Does anyone have any idea what this could be?