My boyfriend and I occasionally have unprotected sex, I'm on birth control (I know it's not 100% percent effective.) Anyways, I didn't get my period last month and I've been keeping an eye on it. A couple days after my missed period, I had some lower abdominal pains when I laid on my side and turned in general. Then the cramping started so I expected to start my period. When I went to change the pad it was a really light red color and later on in the day it turned into a brown almost tar like discharge. I keep getting clumps of tar-like discharge, it's a really dark brown. What could this be?