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.