You're stressing far too much. Clearly you know there's no chance you're pregnant, not only were you on birth control but you've had a period since you had sex. And the nurse is right, there's no point testing for STIs before 2 or 3 weeks, and if you had gone to the GUM clinic they'd have told you they would test you but would have to have you come back 3 weeks later to repeat the tests. Just go to the first appointment you can get as it's doubtful anything would show up before then anyway.