The backdrop is lit so brightly lit that the light spills over onto and washes out the edges of the little girl. Her face and the whole side facing you has ended up underexposed, so underexposed that I'm guessing you had to brighten it quite a bit in post processing that it has caused the skin tones to look unnatural. So expose for the subject, and adjust your lighting on the backdrop to just take it to white.
If you explain your scene and lighting set up it will be easier to give help. Did you use flash or is that backdrop blown out by some other light source?