Is your bird overweight? I ask this because it seems that you are hearing the squeaking sound when she exerts herself. Just as in people, if someone that is not physically fit exerts themselves it might result in labored breathing and wheezing. You said that she struggles when you try to listen to her breathing, are you holding her in your hand, if so make sure that you do not have your hand wrapped around her body so as to compress her air sacs, holding a bird incorrectly could kill them if the hold is not allowing free movement of the chest. It may also be that your bird is developing a respiratory problem, a visit to an avian vet would give you more info as to the exact cause.