Coworker keeps buying me drinks?

I have a coworker I work with, that's become a bit more chatty then she was in the past when we work together. It seems like lately she's been offering me one of her monsters she buys, or like last night she insisted on buying my shake when I offered to pay for one multiple times when I gave her a ride home. I was slightly uncomfortable with it, how I was raised.
Coworker keeps buying me drinks?
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