In my experience, I wouldn't try to over analyze the situation because you don't really know the person well enough to speculate on any ulterior motives. I'd accept this situation as it appears on the surface: a random thoughtful action that arose from positive motives. My response would be to thank her for her thoughtfulness, wear the item at least once in her presence out of politeness, and leave it at that. I wouldn't respond in kind, unless you want to start picking out clothes for each other in thrift shops. I'd also treat her to coffee, or lunch, spontaneously, at some future date without tying it to this gift in any way.
I've been in somewhat similar situations and, unless the gift is clearly inappropriate or I'm pretty certain of the motive behind the gifting, I try to see the situation as a random act of kindness and thoughtfulness.
