What Causes Spotting Before The Periods While On Birth Control?

I have been with my bf. for 4 yrs now. we have unprotected sex but I am on the pill.. In April when I was with him i missed a pill and took them at different times of the day. I started spotting before my period pills and when it was time for my period I was still JUST spotting. Now I'm almost at the end of another pack and im starting to spot again before my period pills.. I also recently changed birth controls. could this have anything to do with whats happening to me?