If the e-mail is your only communication, I'd say you have a casual acquaintance, not a friend. Just don't write back next time, see if she mentions it. She's inconsiderate and oblivious. Maybe she just doesn't know what to say as a response to your question. Or - She isn't very social on her computer.
Some people are still very wary about releasing personal information of any sort over the internet.
Or - Maybe her internet service was down, and she just finally got around to getting it back in working order. They kind of get online when they are totally bored with everything else. I say different strokes for different folks. You can't be too good of friends. Unless your friend doesn't open her e-mail or go on her computer very often.