What Causes Black Blood Before And After A Menstrual Cycle?

I read most of the responses about black blood being old blood at the end of a menstral cycle. But what if it is before AND after my cycle? I don't think it is dark red because it has stained my underwear black and I can no longer wear them. I also get cramps that are so bad, I only have enough energy to change my pads and go bakc to bed. Please help!