Scared!

I had sex for the first time with my boyfriend last Thursday and I had a good experience. We have been having sex this week also. They all have been un protected but he finishes by his own. I was 3% fertile and my period was supposed to start yesterday and I am not regular but this has me out of my mind. I am planning on getting a contraceptive tomorrow. I feel small cramps and my boobs hurt a bit, but since these were my first times I am really scared and nervous. Is there a way I can be pregnant? or how long should I wait?