I get brown discharge at the beginning of my period. That's how I know it's on its way. If i have sex the day it's due, I'll sometimes have a bit of brown discharge, it's like the sex jump starts my period or something.

There could be 3 answers to yoru question.

1- Implantation bleeding, Is this with in 5 days with in having sex, where it is light red or brown clumps, this means your pregnant.

2- Misscarriae, although unlikley, this may not show until 3 -5 weeks after having sex

3- You just passing out natures business, this happens very often to women

Look here