Am I over reacting or do I have a valid reason to be worried?

I was hanging out with my fiance the other night (who is a mechanic) and he showed me a picture on his phone. While I looked at his screen, a notification from Snapchat popped up with a chat and the person's name...I said to a him "that better not be your ex girlfriend" and he told me is was her, she needs her car worked on... I am okay with him working on her car,that does not bother me as he is receiving his income and providing for his family. What I'm not okay with is that he never told me anything about it, not even a heads up like "hey so and so is going to be coming by the shop to have her car looked at. I just wanted to let you know ahead of time." Instead he used to Snapchat to talk to her, and was talking to her for God only knows how long,and she stopped by the shop, all while I had no idea..and back to the snapchat part of it, i'm sure you are aware of Snapchat and how anything you send is deleted automatically... He stated it was all work-related, there was nothing going on, no sketchy stuff, no other conversations other than repairs to her car. but on the other hand I can't help but think "why is he keeping it so private?" and I would have never even known about any of this had I not seen the notification at the time that I did when I looked at his phone. I'm not sure if this is something I should be concerned about or if I'm just overreacting. he says I'm making a big deal about it I just feel as though out of respect I deserved a heads up just to let me know. Yeah I probably would have been a little upset at first but I would have gotten over it. The way he went about things just made my anxiety worse and is now making me feel like he's going behind my back and talking to her and seeing her.