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Whether or not you're over thinking it would depend on how much time you spend ruminating over it and his motivation behind it. You may want to see it from his perspective. Did he do it to protect you? Because he promised her or her husband, he wanted to help this situation himself, etc.
Just because he kept this a secret does not mean that he's done other s behind your back. You're making that association because of your own reasons i.e fear, intuition, etc. But, these in themselves do not have to be connected to his actual behavior(s) You know him better than anyone else. If he has a history of deceitfulness, it is most likely he's doing it now. If this was a one time incident and he had reasons for it, then focusing on it too much may only cause you more anxiety unless there is proof.