[Michael Romanos]

One thing for sure: something has set your hitherto love-everyone Corgi into a sad, sad spin.
It is not age. Is it changed circumstances that have stresed and confused her to breaking point.
She doesn't give out herding nips because such nips are usually just that - a nip rather than a bite.
It is something medical. I would think it most likely is. She could be suffering from something as yet undiagnosed.
Debbie has the steps in order - complete medical check and if that can't dig up a cause, then a Cesar Millan. Don't give her up. Vastly most people and most authorities don't understand dogs and don't want to.