So first, if they thought it was herpes, they might have asked your mom to leave the room to tell you. Doctors, by law, are not allowed to not tell adults - and you are one at 18 - what their diagnoses are, so if they thought it was herpes, they can't not tell you.
I'd be thinking strep or something way before I'd be thinking herpes, for sure, and rapid tests aren't always accurate. Did you tell the doctor about the hook up with the guy? Did the doctor know that you were afraid an STD might be a possibility?
When you had the hookup, did you give the guy oral sex? If not, there's no chance that this is an oral STD. It could be oral herpes if you kissed him, but I'd think any number of viral things that cause sore throats first since you work with kids. It's still cold and flu season.