How should I interpret this? F35, M40

Please read this scenario and let me know your thoughts:

Sitting on the couch together. As we watch tv I feel my back and notice it is getting muscular and toned. I (F) have been working really hard at the gym, especially upper body.

I say to my boyfriend giddily and proudly, “I was just rubbing my back and realized how different it feels! It feels so toned and muscular. It’s so cool!”

After a 5 second pause he says in a serious voice, “Have you never worked out before?” (The tone was not a curious one..) He has been to the gym with me and knows how hard I’ve been working on it.

I know how I feel about it but I want to get an unbiased opinion. We have been together 5 years.

Thank you!