What Causes Light Bleeding Instead Of Normal Periods?

im on birth control i have had some problems with it. I took my pill to start my period on sunday and my period usually doesnt start tell wed. On wed it somewhat started but it was really light and pink in color my period is usually realy heavy and red. Is this implantation bleeding or just a regular period? i took a pregancy test and it came back negative but i wasnt sure if its implantation bleeding if it would read right or not.