Let's hope it's not stomatitis. Often it's not, but a mouth injury can eventually lead to stomatitis, tooth loss, or a deeper infection of the jaw bone. Personally I used Hibiclens diluted with water, twice daily. You have to use a swab and sort of roll it up under the lip. Don't worry if it bleeds a little the first few days of treatment. That's OK. The active ingredient is Chlorhexidine gluconate. You can feed the snake, but treat/clean the area immediately after feeding. Keep the snake in a container with paper towels and keep very clean and dry during the treatment. Keep up the treatment for about 5 days after it heals, and check daily to make sure swelling isn't coming back. This will clear it up in about two weeks. Works for me anyway, for simple injury. Don't know if it will work for stomatitis. Doesn't sound like stomatitis to me anyway, if there's no crusty yellowish or "cheesy" looking buildup. What you described happens fairly commonly, and is probably just a minor injury to the gums from food fights between snakes, or some other injurious incident, followed by minor infection and swelling. Once it swells, debris gets in there and can cause infection. At this point, peroxide is better than nothing, but 1-2% solution of Chlorhexidine gluconate (twice a day, avoid using excess and avoid swallowing) is much better. It won't injure healthy tissue like peroxide can.

Start treatment immediately. The sooner you notice and start treatment, the better the prognosis.