Ok so first of all stop and breath. Lets look at the facts -
You were at risk as you had unprotected sex. This is a fact. Was she a friend? sex worker? Did you talk about STIs or anything? (assuming shes a friend and you trust her). You say guilt - im guessing you have a partner at home? Remember as AJ says, guilt does not equal risk.
So heres what you need to do. Get a test at the correct times. The times are...
Chlamydia - 2 weeks
gonorrhoea - 2 weeks
HIV - 4 weeks
Syphilis - 6 weeks.
Syphilis is uncommon in developed countries and HIV is also uncommon. The majority of people do not have any STIs so from a statistic point of view the test will most likely be negative. The two common ones (chlamydia and gono) if you went to test now, the result would be 95%ish accurate.