What are the chances that I am pregnant?

My period is irregular and I haven't gotten it for three months. However, yesterday, I had a little bit of spotting and then it stopped on the same day. Today, I had sex three times in a row. The first two times we used a condom and the last time we didnt. He says that he is probably sterile because he got denied from a sperm bank two years ago and has been smoking weed for 8 years. What are the chances that I could be pregnant? I can't ask my mom for the plan B pill because I would get in alot of trouble. SOMEONE HELP PLEASE?!