Let me just pop in here to recommend the book NOT "Just Friends" - I have a zillion sticky notes in that book. It TOTALLY starts as 'just friends' but the boundaries are blurred. If you can't share all that you're doing/talking about with the opposite sex person with your spouse, something is wrong.
My H had this very same thing happen and he details the start of his slippery slope into his A here - I HOPE you can't relate to it, but I think you may.
Forget the former things; do not dwell on the past. See, I am doing a new thing! Is 43:18-19
If it seems slow in coming, wait. It's on its way. It will come right on time. Hab 2:3
