Last month i had burning sensation on my right lower abdominal followed by faintly positive upt and next day negative beta hcg and nothing on ultrasound. I started bleeding and cramping  on next day and it happened for 2 days, stopped bleeding and cramping after that. This month ( no intimacy this month)  i was late by one week and took a upt and it was faintly positive, did a beta hcg and it was less than 2 hency negative. My periods came out heavy and with lot of clots, i got ultrasound done on 2nd day and nothing was there, no pregnancy anywhere, no mass, no rpoc or any fluid. Bleeding stopped after 3 days and my test is still faintly positive. What is causing this?