Help! Am I pregnant?

So I'm kind of freaking out. So I had sex for the first time in 5 months the other week & when I had sex, I was already 4 days late on my period. I wasn't concerned because like I said, I haven't have sex in months. We used a condom, but now my period is 10 days late. I don't know what to do! I was going to buy a test, but what could this be? Should I be worried?

Thank you!!!