A. Constipation is usually caused by a child's diet and it is made worse when it hurts to have a bowel movement and they hold it in. So it is likely a combination of those two factors.
When treating constipation, it is usually best to try and change a child's diet so that it includes more fiber, bran, and fluids, and avoid constipating foods. The fact that she eats a lot of junk food, which is likely low in fiber and high in fat, and therefore constipating, is likely a big part of the problem.
Does she drink a lot of whole cow's milk? That is another common cause of constipation, especially if she drinks much more than about 16 ounces a day. Switching to low fat milk, soy milk, or finding another source of calcium, like orange juice, can also be helpful.
If you are having trouble changing her diet, then she may need a stool softener on a regular basis before she starts having regular, soft stools and doesn't fear having bowel movements anymore.
And if she is already having soft stools, it can be that she just remembers that it hurt and so still afraid that it will happen again. Keeping her stools soft and continuing to encourage her (but don't force or punish her) to use the potty to have a bowel movement a few times a day often helps this problem until the child outgrows that fear.
For more information, see our constipation category, with information about the diagnosis and treatment of children who are constipated.
