Why is it that one gets a cramp?

Uterus contraction: The inside lining of your uterus grows in response to female hormones, and sloughs during a period releasing hormones called prostaglandins. They make your muscular uterine walls contract. Drugs like Ibuprofen & Naproxen block production and reduce pain; best taken at the earliest onset of cramps. Also birth control pills reduce growth of the lining, hence reducing cramps. Heating pad too.