AIO. Found an Old Text on My Girlfriend's Phone and It’s Been Bugging Me

I've been dating my girlfriend (she's 21) for a over a year, and everything's been great.

Today, while scrolling through old messages on her phone (with her permission) I saw a text from a guy I don't know. Said "Still down to hang out this weekend?"

I asked her about it, and she said it was just an old friend. no big deal, and she didn't even reply.

She's never given me a reason not to trust her. but this text feels off. Am I overthinking or should I be worried?