What Causes Inter Menstrual Bleeding?

Eleven days after my period stopped I began to have some bleeding. The blood is dark brown to black, and my stomach has been feeling odd. I've been using a tampon but there is still some bright red blood when wiping. I've had yeast infection like symptoms but no discharge. My last period was mostly normal, it started the same as usual. But it went from heavy to light to heavy again and that isn't normal for me. Is this something that I should be concerned with?