My ex is seeing someone, not sure why this bothers me

I broke up with my ex after being together for almost 8 years approximately half a year ago. First few months have been rather hard, I did go through insane depression, but slowly managed to crawl out of it. It really does get better. I hit the gym, started going on dates and generally started feeling better about myself.

I recently talked to my ex over Skype (we are on good terms) and she told me is seeing someone (someone I know). He is a great guy, and I am happy for her, however every time I think about it (this random thought comes up pretty often), it really depresses me and affects my mood and hence everything else.

I have no idea why this is so (I really don't think I have any feelings for her). I know it's generally recommended not to have any contact, but I thought I was ready.

Can anyone offer me any advice? Very much appreciated.