I'd suggest that you start keeping a record of your menstrual cycles, including when menstruation begins and ends, how much flow you have (count numbers of pads and tampons used, noting whether they are soaked), and any other symptoms you experience.
In fact, heavy bleeding is more worrisome as a symptom than light menstrual blood...heavy bleeding may be related to hormonal abnormalities, and treatment of the specific abnormality will correct the bleeding. I think that your symptoms simply might indicate a change in flow after birth control...but if you suspect something else, I suggest you see your gynecologist and take your mesntrual record with you!