What Causes Inter Menstrual Bleeding After Taking Pills?

im am just wonderin, since i started my time of the month when i was 14 (i am now 19) i have never had any problems, now i have been on the pill the past two years and never had any problems either i take it for 3 weeks and then have the 1 weeks break and my period happens but this month have way though taking my pill with around 2 weeks untill my period is due i have started to bleed for 3 days so far heavlyly?