Texting a girl he use to sleep with

Ooooookkay so my husband and I split up for a few months about two years ago and he did his thing... cool no big deal! BUT I found a message in his phone when I was getting our realtors info for our home loan (going over the checklist for inspection) when I found a text from a girl he slept with during that time. Didn’t mention he’s married. Didn’t mention a baby due the end of this month. She’s trying to see him on Sunday (his off day) and he said he had plans (it’s our only off day together)... well apparently they ran into each other and the whole “it was good seeing you the other day” when he mentioned none of that to me! Now I’m sleeping on the couch trying to find the best way to approach this topic... advice??