She is constipated. Take a close look at her diet. Too much dairy and refined food is constipating. She needs plenty of fruits and veggies and high fiber bread and cereal.
I give my kids All Bran yogurt bites, and they love it. We buy double fiber bread from Oroweat. Plus lots of fruit and veg. I also notice my kids are highly irritable and don't behave well if their digestion is not optimal.
Also check out Dr. Oz at RealAge for great info on how to improve health through diet. Get the book The Owners Manual of YOU. Frankly I am shocked at how many parents chimed in on this problem. If your child is starting out life this way, the health ramifications going forward are so serious. Once that colon is stretched out too long, it will NEVER go back. Plus, all of the other health problems that arise from an unbalanced diet.
The way I improved my kids diet, and my own, is that we simply don't have refined junk in the house. They make good choices, because they are the only options we have. The kids will eat all the good stuff, if that is all that's available. And then their tastes will change and they won't want the junk as much. My kids wolf down celery and carrots, tons of fruit, and love it now. We don't miss the junk one bit now. And we all feel so much better. I noticed my kids started behaving so much better when the sugar went out of their diet.