I don’t know, but I don’t think it has anything to do with the food.
Example: http://www.dog-health-guide.org/doglumpsonskin-1.html
“Dog lumps on skin (also called dog skin tumors) can have many causes such as an insect bite that results in an abscess where pus forms under the skin, lipomas (fatty tumors which are usually benign or not cancerous), skin tumors or cysts. Don’t rush to judgment and assume that a canine skin bump is cancerous, as many types are benign or not cancerous”.
Let us know what the vet says. Good luck
