What Causes Painful Bowel Movements In A Child?

My 2 yr old daughter makes her body rigid and sometimes goes on her tippy toes when she is doing a poo. She is still in nappies, has no interest in the potty. She hates me changing her nappy. I have to do it standing up and she clenches her bottocks making it difficul to clean. She doesn t like me wiping but doesn t mind me putting cream on. I can see that her bum is a little red. I think the pooing is linked to a bout of constipation she had and must have experienced some pain when pooing and now associates pooing with pain. I think she sometimes trys to hold it in as when i go to change her there looks like she hasn t quite finished if you know what i mean. Don t know if it is all psychological or a medical problem? Any ideas?