My 16 week old kitten has a fever, won't eat, is limping and lethargic. Please help!!!

My husband and I recently took in three kittens for a friend who has rescued them. One of them has already been rehomed, one is still looking for a home, and we decided to keep the calico kitten. All kittens have been playing and eating together, and using the same litterbox.
Recently our calico kitten, October, has become sick. She started limping, stopped eating, and has been sleeping all day. She doesn't have any sores, redness, swelling, runny eyes or nose and didn't injure her paw in any way. We are keeping her in a separate room, away from our other two adult cats (both fully vaccinated) and our dog and the other kitten.
The vet said she had a fever, gave us antibiotics for her and gave her IV fluids. Blood tests all came back negative so the vet wasn't able to tell us what's making her sick. The other kitten in our home seems healthy but my friend who adopted the male kitten (named Braveheart) started limping last night! None of the kittens had been vaccinated yet. We had just gotten them days ago and haven't had a chance to take them to the vet.

What could be wrong with little October and how can we help her?

Here is a picture of our sweet little October. We love her so much!

She needs to get back to the vet ASAP.  The stress from being in a new home has probably caused her to have an upper respiratory infection.  It's very, very common.  If she's not improving, she needs to go back to the vet.  How long has it been since she's been to the vet?

We took her to emergency vet last night, this is how she was then and he couldn't give any answers besides telling us to do what we are doing.. May try a new vet.

Okay, so it's only been overnight?  What antibiotic did he give her?  I'd give them 48-72 hours to take effect.  If you don't see some kind of an improvement tomorrow, I'd definitely be more concerned.  Keep her warm.  Be sure she's eating.  If you need to, you can syringe feed her.  But, she MUST eat or you'll be dealing with a whole different situation.

Yeah she ate some this morning, not as much as I'd like though. I can't remember what antibiotic, I'll check when I get home. She's started shivering horribly too

Should I be worried about the other adult cats and the kitten? They were all playing and eating together until October