Brown blood is old blood. It can even be old period blood. Or if your birth control pills are perhaps doing something like causing a bit too much progesterone in your system, that can cause it. Unprotected sex in terms of pregnancy is okay when you are on the pill (not for std's however). I would see if this pattern of spotting continues and if it does, see your doctor. good luck
Sounds like some mild spotting, the blood can look brown sometimes. I used to get this if I didn't take my birth control pills at the exact same time every day. If it happens more often or if you are concerned maybe ask your doctor for s different pill.