So about three days ago I had unprotected sex with this guy. I am on birth control and haven't had issues after sexual intercourse before. We have had sex before and don't tend to use condoms and he has finished inside me before but this last time about three days ago I have had odd brown discharge in my underwear. I missed a birth control pill around 20ish days ago and wasn't sure if this could relate to it.  Any ideas as to what I should do or what it could be? Thanks.