OIH! You can't just tell us the start of this story and then stop there! Gah! If I was in Japan, I would've bought the tankobon, but there are no options for that here!
Suddenly the misunderstanding makes a lot of sense. Soujirou thinks that Chiyo has had a forced smile this whole time, and he was right about it being strained at the start.
She had the same expression the whole time, he just didn't notice when the reason behind it changed.
I feel like that's reasonable, since they've been married a month at chapter 1.