Lethargic need advice

Our sweet ferret is 18 months old. She has been lethargic and inactive for about a week. I took her to the vet 5 days ago where she had an X-ray that showed no blockage and her glucose level was just fine. No hair loss, eyes look fine, and she doesn’t appear to be in pain. The vet gave her an antibiotic which she is still getting. She eats and drinks fine, but then just lays and sleeps and when we hold her she is pretty limp. She walks very little mainly just go find a place to curl up in private to sleep. 
Any recommendations. 

Ferret Food
The ferret is a carnivore, and the basis of its diet should be protein food.

What to Feed Ferrets?
Feeding ferrets is an art. If you want your ferret to be cheerful, healthy and active at all times, proper and balanced feeding is extremely important.

Improper feeding of the ferret leads to the development of chronic diseases, some of which may not appear until several years later. Therefore, the common argument of some ferret owners, “I’ve been eating this food for half a year and feeling great,” cannot be an indicator of proper feeding.

Ferrets are strict predators, in nature they feed mainly on rodents – mice and voles, as well as small birds, chicks and eggs, frogs, large insects, fish, carrion. Ferrets eat the caught prey whole, meat, entrails, some bone and connective tissue and wool get into their digestive tract. The only plant-based food that enters the body is the contents of the victim’s stomach and intestines (semi-digested grains, herbs, fruits and vegetables).

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