Black specks when I urinate?

I've been having flank/abdominal pain for months. A lot of doctors/specialists dont think it's anything. About 2-3 weeks ago, I've been seeing black specks when I pee. It does burn a little bit when I wipe. I do get bad bladder cramps, flank and abdominal pain, and nausea. What should I do? I'm not active either.