The last two times after I've had sex, I've gotten my period. In the last month, I had a period November 3rd, which was my usual cycle. A couple weeks later, I had sex and got my period again the next day. It was a typical period for me, lasting 4 days. A week and a half later, I had sex again and once again had spotting for two days. I am on and off with the pill, remembering some days and forgetting the next. I am extremely worried that I am pregnant regardless of all the bleeding, although I've had a negative test. Is there an explanation for what I am experiencing?