Pregnancy sex and cramps

My boyfriend and I had sex this morning and it was good, great even. But EVERYTIME halfway through my lower stomach starts to cramp and the doctor said it was normal. But this time when I had an orgasm I was prepared for the cramping afterwards, not for something I would consider a contraction.  It hurt so bad I cried and I felt the sensation to push and it scared me. I told myself I needed to relax and take deep breaths. I took a warm/hot shower to ease the pain and it helped tremendously. I just wanted to know if this has happened to anyone else before?