Has she possibly been going through something stressful or emotional? I know some people will push others away when they're going through a tough time. Or if you're the one who has been going through some stuff, sometimes people don't know how to handle it, don't know how to be supportive so they run away.
If nothing like that has been going on, I don't know what to suggest except maybe she's just changed - and not for the better.
That's assume that when she became distance, you reached out to her and asked what was wrong with concern and that during the email exchange, you didn't say anything hurtful.