My girlfriend is 21 , we had sex for the first time 30 days ago, protected clean, 7 days ago she had a marathon, yesterday she had heavy back and lower abdomen pain since 3 days, now she has lower abdomen pain what could it be, periods are to arrive after 9 days as per cycle, pain is more like heavy menstrual kind, sometimes mild and persistent, only in evenings , little sensitive to cold and weather is getting colder, also it increases when she mastrubates for longer duration, no sex in this duration