Just be honest with your doctor. It's confidential, and they aren't going to think any less of you for saying you might have a risk. Actually, they'll probably be impressed with your honesty because there are alot of people who would rather lie about that sort of thing to avoid percieved embarassment.
Tell your doctor that you had unprotected sexual contact with someone who may have had an STD and that you need a full STD screen. You're going to need to go back again in 6 months to be screened again, and then again at a year since some STDs can take a while to show up. Also, you'll need to be extra vigilant of your health since you may be carrying something and not know about it. Keep in mind from now on that STDs can happen to anyone, and as much as you want to trust somebody, you simply shouldn't when it comes to being "clean". STDs can happen with or without penetration (anally, vaginally, or orally).
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