Sometimes close friends are friends for life, even if you don't spend lots of time together. She may have been hurt because she felt like you were her closest friend still, and had been through the years even though you hadn't spent as much time together as friends are expected too. I would go off to school for months at a time, go a year without seeing a close friend and seems like it's a bond that won't be broken.
That being said she should still at least understand why it feels odd to you that she asked. She may have not made any better or closer friends over the years, but how should you know that if you are not as close as you once were. Unless you have some special MOH skills she's aware of it's safe to say she chose you because she thought you were closest to her.
Saying no because of lack of funds, shyness, and what have you is acceptable and she seemed to accept and understand. It is. By being surprised she asked though you were saying something about her though, either that you weren't as close as she thought you were, or that you would have thought she would have made closer friends by now even though she has not. I'm sure you don't see it that way but everyone looks at things from their own perspective. You never know what's going through someone's head.