Given the choice between a Nylabone, a real deer antler, and a sterilized bone, Noelle chose to chew on the plastic cup. You'll notice that wasn't in her original list of choices - but that is probably what makes it so much fun to chew on.
I believe this is Noelle practicing her best "who me?" look. She may be suggesting that she has really been busy chewing the deer antler under her front leg, and the cup just happened to end up right there in front of her somehow.