Birth control question

So recently I've started birth control. This is my first time taking birth control so of course I read the instructions carefully and made sure I understood it properly before I started. I started my birth control on day 1(when my period started) and everything went well until I missed three days on the third week. I was helping my sister move into her new house and we ended up staying there for the three days without notice. So went through the three days and I finally came home and took my pill of that day late. I had sex (protected of course) and then right afterwards my period started two weeks early. It's lasted for four to five days. What happened to me? I'm very confused as to why this happened.