Did my husband cheat?

It was late and I was awake with our first newborn baby and his phone kept going off. We have never had struggles before, we've always been so happy and in love. We are about to celebrate our 2 year anniversary in February..Never thought he'd ever betray me in any way so when his phone kept going off I didn't think anything of it. I was trying to figure out why it was going off and I ended up on his Facebook and his messenger opened and there were messages... him and a girl he use to fool around with before us had been sexting. She sent nudes, he sent nudes. Telling each other what they wanted to do to each other.... is that considered cheating? I just had a baby and now I feel so insecure. I can't compete to her. Not in any way. I just feel so hurt that he did this to me.