I've had brown discharge or what looks like old blood for 2 weeks now. I'm 21 weeks pregnant and have been to the dr. All looked good. What could it be?

Need more evaluation. Hopefully you have had a level ii sonogram to look for placental problems which is the most serious problem associated with bleeding at this stage. Next you should hae an exam to check for polyps and infections by doing cervical cultures. Sex can also cause some cervical bleeding.