Jealousy

I hate getting so jealous when it comes to my fiancé! Like every time a girl comments or he becomes friends with one on Fb, I get so mad. Like who is this girl commenting on his posts. If I were to ask, he will say its someone he works with and then ask why am I getting so jealous and stuff.
He has never done anything to cause this feeling. He has never cheated on me or anything. I wish I could stop.