I accidentally asked my coworker out out?! HELP!!?

Okay so I am a straightforward kinda gal. I dont believe in just waiting around, so me and this lifeguard flirted for a little bit during our shift, and i know when someone is flirting with me. We exchanged numbers, and stuff but never actually texted, so one day I decided to ask him out to a movie. And he told me he has a girlfriend. So I pretend like I wasnt asking him out. I have a shift with him tomorrow, what do i do????? This is so awkward. And I am so mad, because I dont think guys should flirt back if they have a girlfriend.