Cramps after sex....pregnant?

So ive been having cramps after having sex with my boyfriend....worst and longer cramps than when im on my period. I don't know if its because my period is coming, im pregnant, the sex itself or even miscarriage??. The cramps are major that make me get all sweaty and cry, i need to get on my side for them to feel better but the last for atleast 15-20 minutes. They feel as if something is going to come out. The pains are low in my pelvis area. It has happened about 3 times but not in a row, they are probably two-three weeks apart. Does anyone know whats gng on w me? I took a home pregnancy test last night....my period should come any day now. The test came up negative, should i go to see my doctor?