Brown spotting

It has been 9 days now that I have had this dark brown spotting.It is just starting to wear on my nerves a little too much. The last few days it seems really dark"almost black" .I have been cramping slightly but just figured that was normal because of my uterus growing. My bbt is up and I had my progesterone checked.At 7 weeks it was at 24 which is the same it was at 4 1/2 weeks. My hcg was at 7880.7 at 7 weeks.I have checked and it is at a normal level. What can this black/brown spotting be caused from? It is so thick and sort of "crumbly like sand" That is the best way to describe it. My first hcg levels tripled in 2 days, then they leveled off to normal.I am wondering if this could be caused from a blighted ovum from a twin pregnancy or from 1 of my fibroids. Hmmm??? 8 days till my U/S.  The wait is really hard because of this happening. I am starting to get more scared because I miscarried 3 times before at 8 weeks.Today I am 7 weeks 4 days.

My Miracle Pregnancy!