I am 16 and I am sexually active only with my boyfriend I've been with over a year we both have std tests and were both clean I have ben on the pill for almost a month now and recently I had discovered I had b.v (bacteria vaginosis) so I had ben taking flagyl (the generic pill form) I ended my flagyl pills yesterday and today half way threw the day I see a lot of thick dark brown almost black disharge with stringy like membrane in my underwear there's no smell and it honestly looks a lot like poop! Can you tell me what's wrong?

I can only guess - that's old menstrual tissue and fluid. I don't mean a month old, just old enough to be exposed to air so it turns dark, very common.