Can you be friends with your ex?

So I’m dating this guy and he has an ex girlfriend named (made up name) Lia they dated in sixth grade Then broke up I believe but they remained “friends” since my boyfriends best friend at the time ended up dating her after they broke up then Lia broke up with my boyfriends best friend and she has a current boyfriend we’re all in grade 11 and they still talk and I don’t like to say I’m jealous but I guess you can call it that but I’d like to say I’m observant and I see how Lia is around guys even with a boyfriend she’ll hug and be touchy with her “guy friends” and she’ll try to show off her body in front of them and I feel like it’s suspicious for her to be cool with my boyfriend after they dated not only that but also after he tried to get at her because he was “bored” before we started dating again there’s other things that make me dislike her in a way like she’s commented a heart under his picture before and said “that’s my bestie” or she’ll want his jacket since her team (they both play sports) didnt get one like couldn’t she ask her boyfriend? Am I overreacting or what btw sorry if this is confusing it’s just the history is complicated...