Am I allowed to be annoyed?

My boyfriend is best friends with his ex girlfriend and she still likes him, (She kinda told me and few other people that she wasn't over him) He dumped her but they're still close and hang out in the same group. I'm not friends with his group of friends so I don't hang out with him at lunch and one of my best friends told me that he saw him always sitting really close with her almost cuddling and she's always flirting with him. Should I ask him about it or let it go since he doesn't flirt back with her.