What Causes Brown Bleeding?

I am 27 years old, have hypothryroidism, and have been on depo for almost 3 years (will be 3 years in February). I just started noticing brown blood after not having a period for over 2 years. I do have PCOS and uterine fibroids which is why I was put on depo to begin with, to stop my periods. I had tried several other forms of bc to control my heavy bleeding, but they had lead to long bleeding cycles. I'm concerned with this new bleeding out of nowhere.