The pic is a bit fuzzy, but based on its location looks like a skin burn to me. There's a chance it could be some other type of skin infection but I just can't tell.
The first thing I'd do is double check the temperature right under her basking light. Tell us what it reads so we can help you adjust it.
Second thing I'd do is have a vet examine the discolored area. You can pick up some antibiotic ointment at the vet and they can also check for fungal infections too. Most likely it is a burn.