Cheating or childish?

So my SO and I have been together for 7 years and recently got married 2 months ago. Right after we got back from our honeymoon we got into an argument then escalated very quickly into a serious fight (note that we both made mistakes and it was not 1 persons fault) the tension lasted for about a month and he wasnt normal for most of the time but our sex life remained active. Around that time i had realised that he was being very secretive especially with his phone, when i confronted him he said there was nothing going on and that i was just being paranoid. A few days later i noticed it again and realised how his actions have been changing and i confronted him a second time, he again denied everything and said that everything was normal. I was convinced and finally asked him a third time where he admitted that he had been talking to someone he had met off a dating site for the past 3 weeks but thats its nothing and that he only wanted to mentally "escape our problems". We talked through it and resolved the matter and our relationship has gone back to normal but I cant seem to completely get over the fact that it happened especially since its completely out of character. 
If it really was nothing as he says, is it cheating or is he just being childish??