Do your cat have any underlying sickness which you have been managing over a while? If you're not sure, I would suggest you go and run a full scan and tests to make sure everything is well with her.
This can turn out to be nothing serious, but there is no harm in finding out. If my cat was active and it suddenly became dull this is what I will do.The cat's health is compromised. I would suggest taking it to the vet immediately and find out exactly what is wrong.