Ex popped up

So in September my husband and I, who were separated, got back together at our final divorce hearing. We stopped talking to anyone we had contact with during our nine month split and last night a guy I use to talk to messaged me on Facebook, who isn’t even my friend on there since July, asking me if I got my number changed. Message is below:

I didn’t message back. I just left it on read and went on about my day. Fast forward to today, my husband was going through my phone and seen the message and now he is PISSED. And he keeps saying “You do you and I’m going to do me” then he said “don’t get mad when you see me doing me” and I don’t get what I did wrong. I didn’t message the guy back. Should I had told my husband that he messaged me in the first place? I don’t want him to cheat on me. What should I do?