6 week Kitten with a fever, weight loss and left arm joint pain

Question
Hello, I just took in a stray kitten on Thursday May 26th. She seemed very healthy, noisy and pouncy. However yesterday we noticed she was very lethargic and didn't want to play, we also noticed she wasn't using her left arm, it was just hanging there limp. She will yowl if you touch it. We took her to the vet and he said she had a fever of 103.9 as he assessed the joint she was in obvious pain. He prescribed antibiotics and gave her a shot of ortisone in the arm. That seemed to help, last night she was much more playful allthough she still did not use the arm. Today we gave her her next antibiotic and she seems lethargic again and is still not using the paw. We wondering what you might think is wrong.
We took her to a shelter to be looked at on Saturday the 28th they determined her age at 6 weeks she was given worm medicine and weigh 1.37 pounds when the vet weighed her she weighed 1.2
Thank you for your help, Jaclyn King

Answer
If she was not tested for Feline Leukemia, then she should be.  FIP is also a disease of concern if she has a fever that is not responding to antibiotics.  I am not sure what to say about the paw, but sounds like there may have been trauma, nerve damage or a developemental problem, therefore, I would recommend an x-ray.