I can only tell you about my experience. I'm fostering a little girl who was found at about 2 weeks old with no mother. She was hand raised for 2 weeks by a kind lady, then brought into the shelter as she had to go out of town and couldn't look after her while she was away.
Little Smudge was dehydrated, very underweight, and looked unhealthy. In the first week she lost an ounce, and was playing a bit, and seemed happy, but was quite weak.
At this stage I was giving her the tinned food they gave me, which was only Friskies brand, and that's about all she was eating, and was having regular diarrhea. I took her back to the shelter for a check up and they dewormed her then by giving her half a tablet, and gave her fluids, but said given she was losing weight, and it could be some congenital defect and she may not make it.
I was really upset, but determined to do everything I could. I bought KMR (Stage 1), Pedialyte, Chicken Soup for the Kitten Lovers Soul brand tinned food, high calorie cat/kitten supplement, and Bene-Bac, and mixed all of this together (in fairly equal proportions, but more Pedialyte, and only a small amount of the high calorie supplement.
They'd also given me Royal Canin Babycat dry food for kittens 1-4 months old, so mixed that in as well. I fed her a fairly heaped dessert spoonful (kept i nthe fridge, then mixed a bit of water in and warmed up to room temperature) of that 3 times a day, and also gave mer KMR mixed with Pedialyte a few times a day as well.
Each day after that she has improved. More energy, looks healthier, filling out well, gaining weight, playful as playful can be, and generally like a regular kitten. Her stools also hardened up within a day or two, and she hasn't had diarrhea pretty much since I changed her food.
I don't know if it was the worming that helped her, my mush, or a combination, but she went from may not live, to happy, healthy kitten in about 4 or 5 days.
I'm now slowly in the process of getting her off the mush, and getting her to drink water and eat dry food throughout the day and tinned food 3 times a day.
Change vets, let them know what happened with the worming, and maybe try a combination of food like I did to try and get your little baby fat and happy. Is there a chance that the stress of all the changes coupled with the vet trip caused her problems rather than the worming tablet? My little girl was very weak when she went in, and thrived after that!
Note: I just noticed that the little guy is doing well now, so you probably don't need the advice I've just given, but it may be useful for others in the same situation who read this thread