On birth control but had unprotected sex

My boyfriend and I had unprotected sex 2 days ago. We haven't been having sex a lot and this was the first time in about a month and the first time we were unprotected. I do take my birth control everyday. Sometimes maybe an hour later, but never miss a day. But right after we had sex I had spotting of blood. It starts out red and then goes brown. And it also has been going on for 2 days now. Nothing heavy just a little here and there. Do you know what a could be wrong??