I feel like my boyfriend is flirting with another co-worker

I know it's stupid but she's A LOT cuter than me, just a perfect nose and a cute body and she's really funny and shit...... I work w my bf and this girl, it wasn't really noticable before bc he would kinda only talk to her if she needed help but now they're always laughing or talking to each other and I hear him saying all the things he said to me when we first met and he wanted me to start liking him..........

It's like when he's with or talking to ME he's in a bad mood, he's really touchy and if I get a little stressed over anything (I'm a manager) he gets pissed and acts like I'm taking it out on him if I say something like "hold on, I need to finish this before I forget"

But literally WHENEVER he's around her they both are just lit up like Christmas trees.

I'm just hurting and idk what to do. I'm her manager so I obviously can't be acting jealous or anything like that and I can't say "hey step off" but what am I supposed to do???