Whats this brown discharge?

My period is two weeks late. I was concerned of pregnancy as me and a guy I recently dated did it unprotected for a small amount of time (but it could be to long!) He didnt *** in me but that doesn't mean I couldn't be pregnant. I have been suffering from headaches which as off today have been guessed to be mirgranes according to the Doctor. I did a pregnancy tester 5 nights ago and got an error code. I then did another one two nights ago and it revealed to be not pregnant. A few hours a go a light amount of dark brown discharge has been coming out. I'm not sure what this is! Please help?