Weird. What's so hard about her saying what she's doing in a general sense... "working on an assignment", "getting some stuff typed out", "facebooking", "checking email"....? Whatever it was, she felt it was a boundary violation for you to ask her and really overreacted. It's obvious to me that your question wasn't really wanting to dive into the details of what she's doing but to get her attention. She could have very easily brushed off the question with a general answer in a friendly tone.
ETA: But, if this wasn't a frequent behavior for her, I'd let it go as her having a bad moment. It's obviously not personal to you either way.