All due respect here, but I have to agree with others on this one...the CNA doesn't have to show you anything, it's not her job. Rather, it is your instructors department to show you what to do, and how to do it.
I'm certain the CNA didn't intend to be mean spirited about things, but you have to understand where she's coming from. As you stated in your post "it was her patient anyway". Yes, it probably was, just like the other 25 patients she was probably assigned. Do you really think she has the time or the inclination to give you a hand? I guarantee you that's
the reason she let you know it was her break time in the first place.
Try to remember this: when you're a student, try your best to work with
the staff on the floor, regardless of their level of training. It will help you in the long run to make things run smoothly. You may not have meant anything by your inquisition "isn't this your job?" But that, to me, is more of a slap in the face...try to refrain from doing that unless you truly want to piss someone off. I'm certain she knows what her job is and how to do it. She was up front with you and you fought with her on it.