Black, very dark and constant menstruation, light to heavy c

I really don't know how long this has been going on, but my guess would be about a year and 8 months.

For that long, I have had spotting in between my periods, and it is not getting any better. The spotting is dark and thick, and comes with some light cramping. I do have a "period" every month, which is usually a little on the dark side at first, and then gets redder in mid-week. Painful bursts of cramps come and go for about 7-10 days depending on stress levels.

The spotting ceases for about a day, which I think might be when I am ovulating (not quite sure since I am on a constant stream of birth control pills).

I sometimes feel like I am burning inside. This is also being more frequent.

I have had an ultrasound to check for cysts on my ovaries, and the test came back negative.

Can anyone help?