A weird feeling

So I dated my ex boyfriend over 3 years ago now we were talking for about 8 months and then dated for 5. It was my first ever relationship, so when we broke up it was hard for me and was left having a soft spot for him if that makes sense. Him and my best friend were reasonably close atm we were dating but obviously lost contact around when I did with him. Now recently my best friend and I have come into contact with him lately so me and him have been building a friendship now however a few days ago my best friend says oh yeah I’ve been on FaceTime to him for hours and then a day after she says oh yeah so we met up last night and were talking for hours and obviously them having a friendship is not my problem like I’m happy they have become close again but I’m wondering if I’m wrong have a sense of feeling weird about it, like kind of being left out idk I can’t tell what the weird feeling is but anyway I need advice