This is a great question. You need to make an appointment with your primary care doctor
as soon as possible to have a full history and physical to determine the cause of the rash. Anybody who has had unprotected sex needs to be tested for STDs (sexually transmitted diseases), including herpes. Herpes, otherwise known as HSV (herpes simplex virus), usually initially presents as a very painful outbreak of blisters which then ulcerate. Other sexually transmitted diseases include hepatitis C, HIV, chlamydia, gonorrhea, and syphilis, and depending on your history and examination, your doctor may choose to test you for these diseases as well. Another possibility is that if sufficient lubrication was not used during sexual intercourse, this rash may be related to irritation leading to a dermatitis. If this is the case, it will likely resolve in a few days, and you can consider using over the counter cream to decrease the irritation and pain that you are experiencing. Regardless, it is very important that you visit your primary care doctor, and also that you discuss various options for protection during sexual intercourse with your PCP as well as any sexual partners. This is for your own health and your partner's.