blood clotting not on period?

i have 2 pills left in my pack of birth control and i have a dark red almost brown spotting with blood clots. i have no clue whats happening. i remember seeing something about miscarriages and blood clotting, but i dont think its that honestly. im 16 and im sexually active so thats the first thing that came to my mind but yesterday i was at the doctor cause im sick and they did a pee sample for pregnancy test cause they do that every time, and some doctor came in to tell the guy literally sticking a needle in my arm that “it came out negative” which i think they were talking about the pregnancy test. im not sure though. some one ease my over thinking mind please!!!