I have ordered from here before, they are legit. This is powder form, so you have to measure to get the correct dosing. When I have to mix or divide for dosing I mix it in softened coconut oil and freeze it in doses, til ready to use.
https://jedds.com/products/amoxicillin-10-powder
I've no idea why the would not give antibiotics.... that just seems odd. There are rules about animals used for food, but when it's a pet, and most of the time when a chicken sees a vet they are considered a pet, it's not an issue.
Amoxicillin can be used, if you happen to have any, it's out...
Birds in general don't handle anesthesia well, it can be risky. If you are concerned then I would absolutely call your vet and ask them.
Some antibiotics (which I would assume they prescribed post surgery) can also make them seem tired. And her little body has been through a lot. But your...