21 weeks pregnant, recurring bv. this morning after bm had thick brown discharge. no pains all day and baby girl is very active. mucous plug? old blood?

Get checked: Brown discharge sounds like old blood. I recommend you call your doctor and get checked. Rarely the cervix can start to open at 21 weeks without contractions.

See your OB doctor: You should be seen and examined for any problems relating to your pregnancy. Without a full history and physical no way to know what is serious and what is not.