<human>: Provide a brief overview of the text.

Okay, just so you know, I'm not in a relationship. 
    I'm 18 and so is my best friend and the girl I've been talking to lately.
    What happened was I started talking to the girl a week ago and we went on a date and everything seemed to be going great. I'd started developing feelings for her and then today my best friend decided to tell me that he had sex with her a few days before I went on a date with her.
    I'm glad he told me, I'm not mad at him at all. But whenever his name was brought up to the girl (a few days ago), she always changed the subject and I found this odd; now I know why.
    Redditors, what should I do?
<bot>: My best friend had sex with a girl I like a few days before I went on a date with her, and I don't know what to do