What Could Abdominal And Leg Cramping After End Of Periods Indicate?

Hi i am 20 years old and recently i have been receiving stomach cramping and leg pain like i am getting my period. I recently just ended my period so i know that is not the cause of this pain. I don't think I can be pregnant because I've taken several pregnancy test and they say negative and the last time i had sex was prior to my last period. I also started ,y birth control yesterday but forgot to take it today. Can this be the cause of my stomach pain?