Well first of all, I don't think they every were "a thing" as that would imply it was ongoing. It anything, I imagine it would have been once and that's it. As for what I think it was...
Probably wrong, but it's what I'm working with right now.
Also, I don't think that Bella's husband has been gone for 3 years, but that simply she and her husband weren't intimate with each other for 3 years because the spark went out of their marriage and that whatever caused him to leave was more recent, probably something that happened within the last year.