2 possible causes come immediately to my mind.
One is stomatitis (mouth rot) and the other is MBD.
Check the inside of your skink's mouth. Does it look clean and pink (not knowing much about rainbow skinks, I'll assume they have a pink tongue ordinarily), or is it pale? Is there mucus? If there's alot of mucus or unhealthy looking tissue you're probably dealing with stomatitis.
Check the bones that make up the jaw. Are they misshapen in any way? Do they feel soft or spongy? If so, it's most probably metabolic bone disease.
One other thought -- is the skink wheezing or whistling when it breathes? Is there dried or wet snot in the nostrils? Could be a bad respiratory infection.
All of these situations (and proper diagnonis) call for veterinary intervention. I suggest that you call your vet ASAP.
Check anapsid.org, arav.org or herpvetconnection.com for vets in your area if you don't already have one.
Good luck!
-Z