When was she altered and where? Lethargy is not good. It usually means something like a serious infection. Could be anything.
Sometimes, a spay may have been incomplete, and any material left behind can deteriorate and cause a sick dog with symptoms kind of like pyometra. Have you taken her temperature?
But it could be anything. New strain of distemper out there. Parvo. Lepto. Pano. Could be a twisted stomach (bloat) or intestine. Yeah, I wouldn't wait until tomorrow, I would get the pup to the vet today. There is just no way for us to diagnose a dog that is having a marked change in behavior.
Checking temperature can give you information. Checking for dehydration. Checking the gums for refill. The more information you can give whoever you are taking her to the better, including when she had her last shots, and spay surgery, preventatives (flea/tick/heartworm), etc.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.