I F30 am feeling insecure about a social media message between my boyfriend and a friend of his?

Hi! Looking to get another perspective on something I noticed today when I was with my boyfriend. He was going through his instagram notifications and opened up a DM from this girl. According to him they are friends, but they did date for a month or two a couple of years ago. The DM showed a shirt he had sent to her and it had something to do with the fact that she has big breasts, and she in return sent a different one back along the same lines. Nothing looked too suspicious as far as what they were saying, but I still find myself feeling a little insecure that he in the first place would comment on something like that to another woman friend or not. I don’t mind at all that he keeps in contact with people he’s dated but for me this felt a little off, but also don’t know if I’m simply just being too sensitive about it in the first place?

TLDR: my boyfriend sent a meme/shirt about her big boobs to someone he has a small history with from years ago, am I being too sensitive about it?