SO hanging out with a girl

My boyfriend and I were texting tonight while I was at a school event and he casually mentioned he's hanging out with a girl. I joked like "she?????? Lol jk" but it still lowkey made me fidget. I have no reason to believe he's doing anything with her, otherwise he wouldn't have mentioned her. Plus he wouldn't do that to me, as he has been cheated on before. But it still makes me jealous. How do I talk to him about it without sounding like a complete psycho?