Thanks for the additional detal; it helps put your exposure into context. This is a virtually zero risk sitatuion for HIV, which is very rare in UK women (and heteroseuxal men) of your age. Chlamydia is the most likely STD in this situation. Follow my advice about speaking again with her. Whether or not you do that, consider visiting your local NHS GUM clinic and follow their advice about testing for chlamydia and perhaps other infections. But truly, don't worry about HIV.
Welcome to the HIV forum.
Tell me something about the encounter so we can more accurately assess the risk. What country and city are you in? You're male. Is your partner female? How old? Does s/he use drugs by injection? An immigrant from an HIV/AIDS area of the world like southern Africa?
If she is a woman, doesn't have these special HIV risk factors, and you are in the US or another industrialized country, then it is exceedingly unlikely she has HIV; and even if she does, the chance of transmission from a single episode of unprotected vaginal sex is very low, average once for every 2,000 exposures. (That means you could have unprotected vaginal sex once a day with HIV infected partners for 5 years and maybe never catch it.)
The risks for other STDs may be higher, but the odds still are in your favor. You should be on the lookout for common STD symptoms over the next 3 weeks or so. If you don't develop abnormal discharge of pus or mucus from your penis, and no sores or blisters of the penis, you can be pretty certain you weren't infected with anything.
You can also speak with your partner and ask whether s/he has HIV or any other known STDs. You really should do this -- you might find s/he is just as nervous about this encounter as you are. After all, statistically women (if she is female) are at higher risk for HIV and other STDs from their male partner than the other way around.
Finally, even if nothing seems wrong, you can always visit your doctor or your local health department STD clinic and be professionally evaluated and tested. Accurate testing for common STDs like gonorrhea and chlamydia can be done a few days after exposure; for blood tests for syphilis and HIV you need to wait about 6 weeks.
Regards-- HHH, MD
Thanks a lot this is really getting my head straight again.
Thank you very much for the repply. Some more info are that: I am 21 as well as she is. I am currently studying in the UK and she is a UK citizen and student. We both live in Guildford for the time being. I was mainly concerned due to the fact that after we had sex she started talking about her "extreme" experiences she had and I made an (probably a false one though) assumption that she could have respective experiences in her sex life.