dark brown discharge at 23 weeks?

i am 23 weeks pregnant and have been having brown discharge throught my pregnacy...i once wiped pink blood when i was 7 weeks pregnant and went to the ER,there i found out i had BV and was put on the 7 day metronidazole pills. Since lately i've been noticing REALLY DARK BROWN discharge on my pantyliner and i'm a little concerned. I've heard the baby's heartbeat everytime i go to the clinic and i've been feeling frequent movements, plus i've had 2 ultrasound, the first of which was at the ER at 7 weeks and the other at 20 weeks, where i found out i was having a boy.
I need to know if i should be worried at this stage of my pregnacy or what?
I had blood works and urine tests and they said they were normal, and i've done some STD tests (again at the ER) but i don't know if they checked for everything...PLEASE HELP!!!!!!!!!!!