Is it normal for it to sting when you find your ex is in a new relationship?

I had been with my ex (first boyfriend) for almost 6 years. We ended on a good note and remained in touch when we broke up 8 months ago. I have a new bf and we been dating for a while and he’s great. I don’t want to go back to my ex and I think we’re not compatible but I still care for him and he’ll always have some place in my heart. I don’t care about him in a romantic way like that.

Today I found out from him he has a new girl friend when I asked why he didn’t take my calls (when I needed to talk to him about a bill issue we had and needed to fix). When he told me it stung and it hurt. Even though I don’t want him back why does it sting and hurt a lot? I’m sad and I really really like my new boyfriend.

I don’t know why I feel this way.