Jealous

Okay so I hope im not the only one who does this...but my husband dated his ex for five years before we got together. She was one of my best friends when I went to college. My husband and I have been together almost three years now and have been married for almost a year. But she works at our favorite restaurant and it's like every time I see her I get jealous. I don't know why but I mean I get jealous. And confused. I don't know how to explain it really cause I know my husband loves me with all his heart and would never leave me or hurt me but I just don't know why I get jealous? I mean I know beyond a shadow of a doubt that he would never go back to her (cause she cheated on him the whole time they was ever together), but it just makes me jealous when I see her. Anybody else have this problem? How do I overcome this?