What Causes Thick Black Blood After Periods?

I had my period two weeks ago, and today when I used the bathroom there was dark blood on the toilet paper. I went to the OB/GYN today for my yearly exam and she said there was just a little blood left over that it was no big deal. But I'm still bleeding, and it's thick and black. I'm not supposed to start my period until the end of the month and this has never happened before.