Husband was talking to an ex hookup

A couple days ago I saw my husband on Snapchat, which he never uses, and asked who he was talking to. He said it was just a friend who he was giving relationship advice to. Later on at some point we were talking again and he said he also follows the friend on Instagram and gave me her name. I just thought it was a friend. He told me they met at one of his old jobs 6 years ago. For some reason it just felt like a lie because he took so long to answer.

I went thru her IG later on and saw she was 7 years younger than him, making the story about them working together years ago false. I confronted him and asked where he really met her and who is she really. After about an hour of back and forth he finally admitted she was the girl he hooked up with 3 years ago while we were fighting. He told me they weren’t flirting but he had messaged her on IG a couple years back (after we had moved in together) and asked for her number and snap. They would talk about probs in our relationship and say merry Christmas every so often. He was snapping he recently because she needed advice about her failing relationship.

I’m SO upset. He’s making it seem like it shouldn’t be that big of a deal because he claims they never flirted but why keep an ex hook ups IG and message them and chat every now and then?? Why use Snapchat where the messages disappear. He even deleted the messages he sent on IG so I wouldn’t ever see them. He proposed to me. We got married. We’re getting a house together. He told me he wants to have a baby. But all this time he was following and keeping in contact with a random ex hookup he knew all of 4 days. Idk what to do or how to feel. Idk if this is divorce worthy or if I should believe he didn’t flirt. Is this still cheating? Idk :(