I just finished my period last week but i started again yesterday. it's a much darker color than normal. i am on birth control and don't use a condom ?

It happens sometimes: Many women have occasional variation of their mucus discharge, some spotting between periods, a heavier or lighter period, a different shade of color in the menstrual blood, a longer or shorter period, or even a missed period. Having one or two of these symptoms doesn’t necessarily mean there is a medical problem, but a visit to one’s doctor is a good idea if symptoms are persistent or worrisome.