What is wrong with my cat?! The vet didn't know, please help?

There are many causes to drooling and fevers so you may have to dig to find what it is. The lump may or may not be related to the fever and drooling. Sometimes it's the result of an infection or an abscess or an oral condition your cat may have. Kidney and liver disease can cause drooling also. Upper respiratory infections can cause fever and drooling. If your vet didn't do bloodwork, it may be a good thing to ask him about. Sometimes you can isolate issues that way. Just keep her in a warm comfy spot and keep a good eye her. Make sure she's getting some food and water. You're doing great with the canned food and warm water added to it. Good luck!