To get the blood out of your uterus, the muscles tighten and relax. That's the sharp ache you feel between your stomach and lower back.
Cramps can start before your period and last throughout the bleeding process. If they feel mild, like someone's giving your ovaries a solid squeeze, it's normal. Try an over-the-counter medication for pain relief. If they take your breath away or have you doubled over in pain, check in with your doctor.