It doesn't look like a coloration thing to me, nor does it look like a bruise. I'm not certain, but it could be one of several things. Possibly some sort of infection (fungal, bacterial or viral), but it could also be a thermal burn. Not a common place for a burn however. I think the first thing to do is use a OTC antibiotic ointment like Polysporin or triple antibiotic on it to slow or stop any bacterial spread and protect the damaged area. Remember, reptiles take longer to heal than mammals so you may not see much change for a week or so. IF the area does not seem to respond, there is a reddish swollen margin around the patch, or it spreads (take some good photos along the way) you should definitely take her to a vet. Many fungal infections glow in medical blacklight so it might be an easy diagnosis.
