The discharge is brown because it is "old" blood. This means it is most likely from the cervix or the uterus.
this could be "implantation bleeding"- when the embryo burys itself into the uterine wall, or it could be pregnancy spotting. However, it could also be the sign of an STI. Or a result from trauma from sexual intercourse (ie, if he was a bit rough or not enough lubricant was used and you vagina/cervix got chaffed).
The best thing to do would be to go to a doctor ASAP. There are so many different things this could be, but with a few simple tests the doctor should be able to work it out.
Delta: Our paths wind as does our direction. Light shines upon us - we hit pockets of bleak darkness. we come out with new eyes, yet still undefined in terms of whole meaning. How exciting it is to live, love, learn and grow.