You haven't caused anything Hun. Brown blood is old blood, which means there was a very small amount of blood a few days ago, and it's worked it's way out now. Maybe because you were a bit physical (were you more physical than you are in your every day life?) or it may just be a coincidence. Either way, you haven't caused anything. The amount of stress it takes to potentially affect a pregnancy is so much greater; women carry pregnancies through war torn countries, rapes, famine etc. Try to relax. Your cervix is very swollen with blood and very sensitive right now, if you've checked your cervix or dtd in the last few days, this is likely the cause.