Kyla, it’s impossible to say what she has, it could be that the amoxicillin hasn’t kicked in yet and that she’s also fighting off lethargy from the dewormer but it could be something else. Did the vet take bloods and test for FeLv, FIV, toxoplasmosis, calicivirus etc? As always, the best and safest option is to take her to the vet. I’m sorry, I know it’s expensive. Have you contacted the shelter? Will they help with medical attention or do they have something going through the shelter right now? It seems pretty clear that she was sick when you got her from the shelter.
First, is she warm enough (but not too warm)? Does she have a heating pad or a hot water bottle wrapped in a towel (with room to crawl off if she gets too hot)?
Second, is she dehydrated? I’ll post the tent test for you to check and also give you the recipe for home-made pedialyte (you can get premade pedialyte in the children’s aisle at supermarkets). Pedialyte is better than water, as it replaces the vital salts etc that she needs. So give her the pedialyte by dropper rather than water. Nutrical is a high-calorie paste that can help — it’s mostly sugar, I believe, so if you don’t have or can’t get Nutrical, try light-coloured Karo syrup — smear it on her mouth to give her a bit of energy. (No big lumps, nothing she could choke on.) If she won’t lick her lips, stroke a tiny bit directly on her tongue or the roof of her mouth.
Try to get her eating again. Warm her food gently to make it more appetising. Try tuna, tuna water, pure chicken baby food (Gerbers is a good brand) — no garlic or onion or additives as they’re bad for her.
Does she have fleas? They can cause anaemia and death.
But I would start with the pedialyte and just try to rehydrate her gently.