It is most likely an allergic reaction at its peak. Call your vet to make an appointment ASAP just in case; but if it an allergic reaction, the doctor should not be overly worried as long as the cat keeps eating and doesn't seem lethargic. It may be a bug bite, plant, spider....
I also got this from a website. You might want to read it all.
Viral infections - the most common one that causes mouth sores, drooling, and foul breath is a serious infection called calici. This can cause ulcers on the gums, tongue, lips, roof of the mouth, throat, and lungs. This is one of the two most dangerous viral upper respiratory infections. It commonly leads to bacterial infections that require antibiotics to cure. Pneumonia is also a somewhat common complication. Most cats will stop eating when suffering this infection due to mouth pain. Adult cats usually aren't as affected as kittens are, but this is a tough infection for cats of every age and can be fatal if not managed. Hospitalization is needed from time to time if the cat will not eat. Calici is often carried for months, even years, and may cause milder flare ups throughout the cat's life.
The other most dangerous upper respiratory infection that can cause these symptoms is feline herpes. This one doesn't cause sores as often as calici, and it usually produces copious nasal and eye discharge. Treatment is the same as for calici - antibiotics to fight bacterial infection, as well as supportive care with vitamins and subcutaneous fluids if needed. L-lysine is also recommended to use with herpes patients. 500mg a day is the effective dose.
Autoimmune disease is caused by the body's immune system attacking the body's tissues. The mouth is one of the more common places where autoimmune disease is manifested. When it causes sores in the mouth, it's called stomatitis. Stomatitis is especially dangerous because it causes periodontal disease, which may lead to organ failure. Stomatitis can sometimes be managed with antibiotics to prevent infection, or oral steroids to reduce inflammation. Ultimately, most stomatitis patients will need to have their teeth removed. This is because the ligaments that hold the teeth in place are usually the tissues being attacked. Once those ligaments are removed, the disease resolves. I have many cats who are toothless, and they eat both hard and soft food just fine. They actually feel much better once their teeth are pulled.
So definitely seek the advice of a vet. Regardless of the cause, sores in the mouth need to be promptly treated.
Sovereign Silver works great on this....and eye infections, administer oraly 3 time a day, for 2-5 days if being used for any eye infection...keep eye clean with saline and treat with silver..a drop in eyes, a few times a day .....also for urinary tract infections use tinkle tonic...the vets wanted to put my cat to sleep he was so bad....but a week on tinkle tonic and he is doing great 1 year later
How much Sovereign Silver? my sweet kitty weighs about 16 lbs .... the blister is pretty big :( I so look forward to hearing from you Tracey ~ ♥ ~