See, that just makes me think even more that something happened between Jill and Bella. Just that line alone, "And I don't think I can make you understand this without telling you things I shouldn't", makes it sound like Jill is worried that the MC will judge Bella, and possibly even herself, for what happened between Bella and her husband.
Right now, this is still my best guess - Bella's and her husband were having problems in their marriage. He may or not have cheated on her, but they were trying to resolve their differences, although it wasn't easy, and Bella was feeling very sad, depressed, and lonely over the whole thing. Jill had her bad date with Rusty and came to Bella looking for comfort. The 2 of them drank some wine and then Bella offered to let Jill to stay in her bed because her husband wasn't going to be at home that night. When in the bed together, the loneliness and longing for affection they both felt got the better of them and they started making out, possibly got naked as well. The husband came home and found the 2 of them together, then stormed off and he hasn't returned since.