Overreacting about what he said??

Okay so my bf and I have been dating for 4 years and we're pretty open to each other about checking other people out, and such. There's nothing wrong with looking and we both understand that. But today he said something that I felt a little uncomfortable with, I felt like it was crossing the line, but I don't know if I'm overreacting..

I sent him off at subway to get me a sandwich and through the windows I saw him talking and laughing with the girl who worked there, ok, didn't bother me.

Then when he came to the car I was like don't be flirting around when you know I can see you (joking) we talked and joked about it for a little while.

He claimed she started flirting with him, ok whatever, harmless to me.

BUT THEN he said "yeah she had a pretty big butt, like woah big, it caught me off guard and I was like oh damn, I'm not wearing boxers (as we were laying in bed before) and I hope I don't get a boner"

Idk but that hurt my feelings. Lol especially being 8 months pregnant, not feeling my best and I know he likes girls with bigger butts, which I don't have. I'm not gonna lie it kinda made me feel insecure.

Am I overreacting here??

I just got quiet and told him he's taking it to far now and he thinks I'm being dramatic and need to get a grip.

What do yall think??