What you did, you did. Nothing you can undo.
Unprotected sex with unknown and sometimes even known people can be dangerous. Its not a question of 10/15 second or 10/15 hours, unprotected, thats the bottom line. She looked clean doesnot make a change, all you can think of is, if she is fucking her boyfriend and spread to you, then it means she is habituated with others and having more partners too. That is risky area.
For now, dont cry out and blame yourself or did just a bit type of excuse. Go to a doctor, ask for the regular STD checkups and sleep tight. Once the report comes, either be happy or be sad.
Think of this girl as a high class callgirl, what would you have done with a call girl, do the same. Avoid all oral sex, go for protected vaginal intercourse, pay off and sleep tight. At least you are worried about your helth thats good, but knowing these can be dangerous venture still couldnot hold the temptation, that was real bad.