Arm touching from female coworker

There's this lady at work that I've been talking to more and more recently. The other week I had to leave her a note to ask her to do something for me. It wasn't really anything special or anything, but she said it was "super cute" and stuff. Anyway, since then whenever she's spoke to me (and is within reaching distance) she touches my arm, strokes my shoulder or something like that.

What does the whole touching thing mean? I'm not a very tactile person. I never touch people and people never seem to touch me in general conversation. I do like it when she does it, but I wonder why she does it, and if I should try and touch her back? I say try, because I don't know if I can do it without looking really fake or forced!

Thanks :)