Am I overreacting

Maybe I’m overthinking and overreacting but I don’t know..... so I’m in my first serious relationship we are talking about being engaged by the end of this year, but he has an ex that pops up at his job once every blue moon she calls him whenever it’s not often but whenever she feels like talking to him. I know he wouldn’t cheat or cross any lines but it’s the fact that he didn’t tell me she recently came to his job I found out through someone else and I felt really embarrassed... it’s to the point I’m not the type to give ultimatums but I’m strongly contemplating walking away.... I don’t know what to do.... 😩😕