What Causes Constipation And Bloating In A Child?

Hi! My daughter is a little over 2 years old and since she was 6 months (i.e., after she stopped breast feeding), she s had chronic constipation. Our doctor has kept her on Merolax (a stool softener) and swears such long-term use will have no ill effects. Although her stool is now soft, tne problem is that our daughter now tries to hold her stool in (goes ridged and crosses her legs), resulting in something like 12+ small diapers per day and a VERY red bottom, no matter how much diaper cream we use. Also, lately she s complained of abdominal pain a few times a day, and we ve noticed that her tummy is pretty pronounced although she is not at all fat. Now we re worried about some form of bloating too! Any ideas??