im inclined to think maybe she is just a bit lonely, maybe she has a new mobile but has very few people to call or text and as you were nice maybe she just thought shed made a new friend, someone to text etc.
some kids wouldnt see it as odd, and just see you as a nice person, a new pal... maybe she spends a lot of time with older people - aunties etc so sees nothing wrong with befriending older people
also she is coming up to puberty and maybe is starting to wish she was more grown up?
it is a bit odd to us, but in her head it was probably normal.
its more worrying that she may say it to men or younger llads - who i am sure would mostly give the same resopinse you did, but you never know one or two may not, and may see it as a proposition...
getting the paranoid head on...could it have been a cry for help? did she seem at all like something was wrong?