If it was truly your regular period (arriving around when you expected it and normal length and flow), then it's unlikely you'd be pregnant. Most pregnant women don't have a period after conceiving (but some do!). But if it wasn't a real period and was just a bunch of spotting, then that would open up the possibility a little more. Because implantation of the fertilized egg into the womb can cause spotting. Home pregnancy tests are relatively inexpensive and easy, so you could just do one and put your mind at ease. But like I said, if it was a true period, normal length and flow, then the odds would be very against you being pregnant.
Since I've been trying to conceive, I've done alot of reading on the internet and have learned a ton about ovulation and the monthly cycle. There is so much information out there. You should go and do some research yourself, too, so you have a really good understanding of your body, which can be really helpful when trying to get pregnant or avoid pregnancy. (I wish the internet had been around when I was young! Would have been really helpful!) Best of luck to you!
Anytime you have unprotected sex you run a risk of pregnancy. It all depends on when you ovulate. Wait until your next period is due and if it's late or lighter than normal, take a pregnancy test with first morning urine. Unless you want children, I would recommend birth control and condoms.