Ever had a nice act from someone that wasn't a friend or relative?

I just received a $50 gift card in my e-mail from a girl that I’m not that close to. She was thanking me for listening to her and saying it was a belated birthday gift (my birthday was 6 months ago). I accepted it and told her that I don’t deserve this and I will repay her some how and she told me no need to and I do deserve it. I started to wonder what was her motive behind this? Can she be trusted? No one has ever done this for me. We’re not even really close.

Back story about this girl:
I met her boyfriend a few years ago in a class and we became good friends for about a year. We hung out once a month and kept in touch sometimes through text. One day he told me he had to stop talking to me because it was ruining their relationship and she didn’t trust him with me.

Fast forward a couple years later and she finds me on Facebook and requests me as a friend. I told her, “I thought you hated me?” She said she never hated me and that situation was with between me and her bf. She said he was suspicious because he would always lie about talking to me or hanging out with me. I don’t know if that was true but whatever.

It’s been about 2 years since I added her on Facebook. We talk a couple times a month. Maybe I’m naive and easily trust people but I opened up to her a lot about my personal life. She opened up too but I mostly talk about myself because she seems interested. I just mentioned to her yesterday that I wish I had new clothes but Express was expensive and then she asked for my e-mail later and then I receive the gift card.

What you think? Random act of kindness or suspicious?