Fiances ex

When my fiancé and I first started dating it bugged me when his ex girlfriend was brought up. His ex would contact him and try to cause issues when we first started dating. She now works at a store around where we live and I'm nervous someone will run into her and make a big deal about. I say it doesn't bother me anymore so I'm not sure why she makes a big deal about it lol it honestly doesn't bother me and when it's made an issue I get mad/upset. My future mother in law didn't even like her so it truly doesn't hurt my feelings😂 any advice?