What Could Cause Bleeding From The Skin In Between The Buttocks?

After long periods of walking ( such as mowing the yard or going to the store) I find that my posterior is bleeding. The blood is not coming from the anus itself, but rather, between the cheeks. If I had to guess it would be caused by the friction as I walk but I m not sure. Am I right? And if so (or not) how do I stop it?