I hope to be able to help. I realize you are probably really nervous about this situation, so bear with me until I passed along to you all the facts.
Doing research on the latest statistics (CDC), 40,000 people a year are diagnosed with HIV in the United States. Most are under the age of 25 at the age of exposure. Other things that increase the chances of having been exposed to HIV are history of male homosexuality and drug use. I'm only stating these facts because you have to keep in mind that your partner may have had other partners before you.
If you used a condom, that is great. Your chances are pretty much nill of having caught it. The CDC recommends those under 25 and not in a monogamous relationship to be tested at least every 3 years for HIV, and other professional organizations recommend annually if one has had multiple lifetime sexual partners.
What I recommend to you is go and see your primary care provider. If your exposure has been recent, there is anti-viral medication you can take to decrease your risk of aquiring HIV if you are really worried about it. This medication is known to cause stomach upset, but probably your nerves are causing that as well, so it doesn't matter!
Get tested for HIV if you are worried. If you are worried about a recent exposure, it is recommended that you get retested in 3-6 months after the initial test to ensure that you have not been exposed.
Wear condoms. Very important. It only takes a minute, and if you are worried about cost, you can get like 10 for a dollar at planned parenthood or your neighborhood family planning clinic. You can also get your health care provider to write you a prescription for condoms, and you can get them for free at the pharmacy.
Hang in there. I'm glad you are worried about HIV, because maybe the next time, you will protect both yourself and your partner.
Let me attempt to answer your question better. There are an estimated 1,185,000 people in the US with HIV/AIDS. With these statistics, your chances of being exposed to HIV in a single encounter is about 0.003% (less than half a percent chance) assuming you and your partner are heterosexual and do not inject drugs.
Sorry about spouting off other stats, but I wasn't sure if you had other risk factors. But, assuming you don't, your chances are PRETTY slim!
Hope this helps.