If food was stuck there, I betcha its inflamed acne. Warm compresses are a good idea. It will likely go down or "come out" just like when we get a pimple or cyst.
BTW, my girls get occasional acne and they have never ate out of bowl or plastic. They eat off ceramic plates. I think they get food on their chin and if they don't get it cleaned off well it will cause it.
But do keep an eye on it because my previous kitty, Hope, got a bad cyst/bump on her lip and the vet cultured it and it was that nasty bacteria, I forget the name, the one that causes most serious/fatal infections in hospitals. Anyway she did have to go on antibiotics and the vet said it was a very good thing I brought her in cause it really could of gotten out of hand. I am not tell you this to scare you, what happened with her was a really weird and rare thing the vet said, but things do happen.
And after that happened, I was so neurotic, I washed her mouth and chin off after meals. Just like you would a small child, with a baby washcloth. She was such a sweetheart, she learned to expect it and would actually stick her face out to be cleaned like a kitten waiting for mama to clean her after eating.