discharge?? please help

okay so i have a problem.

a week ago, after i was sexually active with my girlfriend, i started getting brown discharge with a strong smell to it. i tried googling and found only that it's old blood from an old period, which i dont think it is, because i had my period normal the month before.

i shouldve gotten my period three days ago but didnt, only the brown discharge continues to happen. and there's a lot of it.

i'm freaked out and i dont know what to do. any advice? please?