I've been 2 weeks late before and not been pregnant. Sometimes our periods "glitch" and hopefully it will be on track next time! And you're not stupid for messing up and having unprotected sex once. We've all been there. Next time you'll just be more responsible because of this scare. Good luck to you!
A pregnancy test would show if you were or were not by now. Stress and a lot of other things can cause you to be late or miss a period and since you are only 2 weeks late that isn't all that unusual. Also, if you only had unprotected sex in September and had a period then your most like not pregnant. Best thing to do is contact your doctor and get checked out to see what is going on.