It's usually caused by the cervix being irritated by sex. This is causing tiny little tears. It's not usually something to worry about. :)
I've had that brown discharge since I got pregnant and I'm now 15 weeks, but I think is normal, I never had it after having sex but I did bled once and I got scared, what helped me was that I put my legs up, so try to do that and maybe it helps and good luck with everything