My period is irregular BUT...

So I'm on birth control, the pill. It's a 21-day regimen with one week of no pills meant for my period. I've been taking the pills since May, and my periods are still irregular. By that, I mean that I get my period during the second week, or third. Just recently, my period has been starting the second week AND the fourth. My last cycle was the first cycle in which it was on the fourth week. Well, I had sex last night and this morning I'm spotting. My period isn't supposed to start for two more Sunday's. What do I do?