so around November 30th I had sexual intercourse, I had pregnancy symptoms and didn't think much of it, then I started my period and the symptoms quit. then after I got off my period, they started right back up. my period was not usual... I went to the doctor, negative pregnancy test, negative blood test, and negative ultrasound. but now my period just went completely missing and I'm having servere lower abdomen pain. any thoughts on what this could be?