What does this mean?

I got off my period on the morning of the 15th. I had unprotected sex on that night. And began my birth control on that Sunday the 17th. Yesterday (the 22nd) I started cramping.. Which was very odd considering I'm not due for my period. This morning I wake up and as soon as I stand up I felt myself ooze. I went to the bathroom to find a dark red glop of blood. What is does this mean?