Late-Reaction Syndrome

What do you call it when a certain circumstance happened and you went initially phlegmatic? A few moments later, I mean days later, you realize you should have shown some reaction.
A month ago, I happened to connect (via text messaging) to a friend whose whereabouts I haven’t heard of for almost/more than six years. The usual “kamustahan” went on, and when the inevitable goodbye came, I said goodnight and told him how nice it is to be texting with him again. He replied: “…Ok, you too.. Just study hard, mahirap ang med. Nyt.” [Just study hard, med (pertaining to medicine) is difficult] I didn’t reply back coz I already said “goodnight” which means the end of conversation. A month later, I found the message in my inbox for it’s not my habit to erase messages immediately and that’s when the un-/intentional implication sunk in. I ponder, why did he tell me that? Does he think I’m not serious with my studies? I should have replied him immediately: “Ya, I know. And, I’m experiencing it down to the deepest layers of my bones.” But, it’s too late na already to and there’s no point of telling him that. It may be a pathetically petty thing, but somehow it has an impact on me.