Could I be pregnant?

I was on the pill (chateal) for a good five months or so starting the end of 2014 going into 2015. My relationship at the time ended and I stopped taking them because it was too much of a pain to take them (and a waste, I felt) when I wasn't having sex. I started taking them again recently. It's Monday now, and I started taking them not this last Monday but the one before I think. They're supposed to take two weeks to work, I thought. Last night I was an idiot and didn't think about it and had unprotected sex. I'm really freaking out. Could I get pregnant? My period isn't supposed to start another week at least I don't think. So tempted to buy some plan b but it's so expensive and I'm just really worried. I know this is a dumb question but I could hardly sleep last night and I'm not sure what to do.