Well, that make me think a little on the story "It happened one super night" by Wendy Richards, but not exactly. In that fanfic, it's Clark who is worried about the father of the baby who don't check on her (he don't remember the father to be him as Superman), and he choose to tell her his secret in her appartment when he go to find out.
So, I don't think that's the story you're looking for, but I can't think of another one. Maybe I just don't read that one. If anybody find out what story is that, I sure will read it: the story of Lois and Superman together, and Lois don't know he is Superman are my favourites.
I hope somebody can tell you what story it is, if it's not that one.
