My ex

Right I need some advice pretty please :(

My ex boyfriend has got in a new relationship. We had such a overwhelming relationship and I've been with my partner for over a year now. I saw a picture of my ex and his new girlfriend on instagram. I work with them both two so when I saw them together it never bothered me but I can't stop thinking about it. I'm not jealous or anything because I'm happy too

But please can someone help me understand why I seem so bothered by this if I'm actually happy myself In a new relationship?