2 (or more?) possibilities:

1) the woman thinks you like her more than just as a co-worker.
she really doesn´t want to hurt your feelings and gave back the gift card because of the reason she already stated.

2) the woman isn´t into you, but feels disappointed about a 10$ gift card as present. even if she doesn´t like you more than a co-worker, she thinks she deserves more than just a 10$ gift card.


- would be interesting to know if you have a crush on her?
- did you talk to her after that?

i´ll go with °1 - and maybe you should make a clear statement on this (if you don´t have a crush on her), as it would be not good for the working climate if she thinks you like her more than you should.
but try to do this in a way that doesn´t make her feel stupid about er assumption.