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.