You need to resolve yourself of the worry. If she decided to send that money, I’m sure it was because she could afford it.
Good deeds always have a way of coming back in positive ways. This was yours. I don’t think I’ve had something like this happen for me before. But it’s understandable because financially, I haven’t been able to help that many people out because of my own situation. I do hope that someone in the future will find my kindness or a good deed I did for them in the past worthy of reaching back out at the least, even if not to send a token.
I couldn't really point out how I helped this person, even if I knew I somehow did. It was 10 to 11 years ago, it's been such a long time and they popping up to actually remind me of kind gestures of over 10 years shows that some people have more sense of gratitude than some others.
I couldn't remember that was why I felt guilty. However, it is a gift from her and it would be more awkward to reject the gift.
Just like you, this seems to be the only person after a long time that has shown gratitude to me.