Without seeing it, it sounds to me as if you've got a mask which isn't rotating when the material inside the mask is rotating. That's the only thing I can imagine which would turn a square into a rhombus. Or are you using a matte channel?
Is it possible that the mask inadvertantly got locked? You can lock a mask independent of the layer it's working on. Just a thought. Otherwise, I would image that the mask always moves and rotates with the clip - I've never had it do otherwise.