You don't have to be addicted to have withdrawal -- some people are more sensitive to this than others. I don't know if it is withdrawal, but the symptoms are classic withdrawal symptoms, but I do agree with the above that it would be uncommon to have this reaction after such a small dose for such a short period of time. But again, we're all different.
I'm not an expert, but I really don't think that small a dose once a day for 2.5 weeks is even close to the point where you would be developing a dependence/addiction. You may simply have gotten yourself worked up, anticipating withdrawal symptoms and having a panic attack as a result. Or it's possible that the Xanax has been keeping your anxiety symptoms at bay. So to answer your question, I don't think you're experiencing withdrawal. I hope tonight goes better, Sophie.