As Dr. Lee says a visit to the vet is in order. Could be many things, almost all easily treatable when caught as early as possible, HyperT being one of the first that came to mind. I would start w/a physical exam, blood and urine tests.
As for cost much of that may depend on your area, since clinics range in prices.
Avoid biting when a simple growl will do
The Spirit Lives As Long As Someone Who Lives Remembers You - Navaho Saying
Vindication ~ For all those pets who became sick or lost their lives from tainted pet food