Asking the real questions

Because in general a long sleeved top will provide you with warmth on your torso and arms but can be loose around the torso area, not good when you're trying to do pirouettes I guess so by wearing a form fitting tank top over the top you'll minimise and/or eliminate swinging in the torso area of the the long sleeved top which will keep her core temperature OK.

Is that Natalie Portman?

Because in the movie, she has mental issues and it tends to make her paranoid. Another example would be when the time she thought someone was following her and would kill her while she was walking down the street but in truth, she was just being paranoid. Another proof that she has mental issues (although not directly related to her paranoia) is when she was delusional that she and Mila Kunis had lesbian sex.

As for clothes: don't judge ballerinas, we make questionable fashion choices