Why is my cat a lethargic, sneezing, twitchy mess?

You need to get this cat to the vet and the sooner the better. Hard to say what exactly is wrong, but obviously something is wrong. The vet will be able to diagnose properly and treat whatever is going on. Anytime there is a change in a cats behavior, activity level, etc...it is reason for concern. Cats often hide illness well, and only exhibit changes in behavior, activity, as a clue that something is wrong. Your cat should go to the vet today for a check up.