All my cats have done this. So, I'd say it's just a generally affectionate behavior. My boy, Obi, will shove his face into the crook of my arm, under my chin or in my hair to purr, knead and drool a bit. Some cats do grow out of it a bit as they get older.
I have heard both the theory they do this if they are weaned too early and that they were weaned to late. I don't think it really matters, I think they do this when they are content and feel safe.
I heard it's called "milk treading". She's trying to find nipples so she can feed. From what I gather kittens do that to the momma's tummy to get her nipples to pop out.
I could be way off - base here, who knows. I noticed that my cats do it when they are getting read to snuggle up & lay down.
Hammie, Jess, Dusty and Zorro all do it; the one who does it the most is Hammie, and I know he was not weaned too early. We joke with them and ask if they're looking for nipples in the blankets!