Betadine isn't a medicine - it's an antiseptic. While it is often used in the treatment of stomatitis, it is done so in conjunction with antibiotics.
Go to the vet and have the bacteria at the root of the infection cultured - there are several resistant-strains of stomatitis floating around. The vet may also find that, depending on how bad it is, that debridement may be necessary.
The bacterium that cause mouth rot are usually already present in the mouth of the snake. Regardless of what has been claimed on this thread, stress can play a critical factor in your animal contracting it.
Stress lowers the ability of the immune system to keep naturally present bacteria levels in check - then all it takes is a cut from live prey, a shed tooth, a breach in the mucous membranes, etc. for the bacteria that is already present in the mouth to overcome the compromised immune system.