Commented on his ex-girlfriend's photo

Last night we went out for dinner. At first, the conversation was good, but then he kept looking at his phone, drifting off. I got curious and asked, 'Did something happen?' He said no. When we got home, I saw that he had commented on a post by his ex. So it turns out he was distracted because of that. Am I overreacting? Or is this really a sign of disrespect?