I dont know what causes it and I dont know if its a "normal" thing to occur after masturbation, but I too have experienced it. I have been on the birth control shot Depo provera for 5-6 years and have talked to the nurse that gives me the shot. She didnt seem too concerned about the cramping, but it does worry me a bit. If you find any answers to our common question, please reply. Thanks Eesh