When you are ovulating your body does two things: releases an egg and thickens uterine lining (so that the "fertilized" egg can safetly implant). The egg implants, fertilized or not. If you aren't pregnant, there won't be a "pregnant" hormone. When there isn't this hormone, the body must get rid of the egg and thickened uterine lining to avoid infection; thus, it is shed. Your uterus is actually ripping and producing blood/uterine fragments under the control of hormones for 3-5 days to get back to normal. That's why you feel off or bad on your period and why you have them.