Help! Miscarriage?

I've been bleeding for 18 days straight. It is mostly brown spotting with clots/tissue. My doctor ordered a blood test yesterday to check my levels. I feel like that should have been done 10 days ago. Would anything even show up if I am miscarrying? I had one very faint positive pregnancy test right before the bleeding started but the rest of the tests were negative. It's driving me insane not knowing what is going on with my body right now. I've been bleeding for over half a month! My periods have never been this long, ever. It's terrifying going to the bathroom and seeing clots in the toilet. If it isn't a miscarriage, what else could it be? I have endometriosis and have had two surgeries for it - the most recent surgery was in February of this year. We've been trying to get pregnant since November of last year. I had the mirena taken out in November as well. And after the first month, my periods have been very on time and lasting no more than 4-5 days. I know we haven't been trying as long as some people have, but it's still an emotional roller coaster every month. I just want answers.