Hi There. Here are some possible reasons why she is sick. You mentioned that she is in a tank. I have heard of some people keeping them successfully in tanks. Especially in colder weather. But the general consensus seems to discourage the use of tanks in favor of screen cages.
Bacteria will build up easier in a glass tank since chams genereally require humidity and lots of water. Bacteria may be making your cham sick.
The mouth sore is hard to see, but check that it isn't crusty food. If thats the case you have nothing to worry about. If it is indeed a sore, it may have been caused by a cricket leg. Sometimes a cricket leg will puncture their mouth. If left untreated, it can develope into a mess. I'd take him to a vet to get him on some anti biotics. There is also a topical liquid you can put on the lip to help clear it up. There is one particular good one that I've heard of but have never used. Its supposed to smell like very strong garlic, and if applied on a regular basis, will help prevent future problems.
In any event, its best to get him to a vet. Once he stops eating, the effort required to get him back to good health is going to be alot more than taking him to the vet NOW.
Peece.
