You might want to read about Post Exposure Prophylaxis. It's a treatment with retrovirals within 72 hours of a possible exposure to HIV, to prevent an infection. I don't know where you are from but if there is public health, they should give it to you for free.
Maybe nothing happens, but there is some risk going raw with a prostitute.
EDIT: As someone said before, the risk for HIV contraction after "insertive vaginal sex", seems to be low. This guide recommends it only if the lady is known to be HIV positive
EDIT 2: Other guy in the same situation as you did receive PEP. While I wouldn't panic about it, as the risk seems low, it still seems a good idea to ask a doctor within 72hs and see if he indicates PEP
Post-exposure prophylaxis, or PEP, is a way of preventing HIV infection. It involves using a four-week course of the drugs used to treat HIV, taken very soon after you may have been exposed to the virus.
www.aidsmap.com