Two Of My Cats Have Developed Very Sore Mouths And

Two of my cats have developed very sore mouths and although they want to eat, and will lap a liquidy diet (ie I mash up the wet food so no lumps and add powdered milk and water) as soon as they seem to encounter a lump they will cry out and back away. I am really worried that it could be a sign of something very serious although they have had half of their cat flu vaccinations. (I suffered a bereavement at this time and did not take them back for the second treatment). Both are desexed and do not spend a great deal of time outdoors