What Causes Irregular Periods Inspite Of Using Pills?

I ve had my period for 9 days. I am on the pill and have been on this specific pill for six months now. I haven t had an irregular period since I ve started this pill. My cycle was supposed to begin today, but I have had my period since Friday (March 28th). I had unprotected sex on that Friday with my boyfriend, before my period had started. Prior to that, I had not had sex since February 27th and started my period March 5th. Should I be concerned?