I'm taking birth control pills and why is my cycle starting in the first week?

I've been taking birth control consistently for four months. Every thing was normal. I had my normal menstrual cycle and it stopped almost two weeks ago. I am now just getting through the first week of a new packet. I started bleeding today after having sex. To be honest the sex was painful during penetration. I also have been experiencing sharp breast pain with slight nipple stimulation, bouts of crying, and food cravings. I haven't got a clue of what is going on with me. I'm hoping someone out there has experienced the same symptoms and enlighten me on what's happening to my body.