Is my kitten lethargic and shaking after flea treatment normal?

It's not likely to be a symptom of the fleas but it could be a symptom of the flea treatment you use, some of them can cause a side effect of lethargy and lack of appetite which usually does not last more than 24 hours. if by tomorrow Jax is not better, i would recommend taking him to your vet to get checked and treated.

This isn't good. While a bath is stressful, it shouldn't stress him out so much that he hasn't eaten for 24 hours and slept the whole time. It's actually very serious when a cat won't eat, so I would take him into a vet immediately to get checked out.

Please have her assessed urgently by an emergency vet and take the shampoo bottle with you. Small kittens are very sensitive to insecticides and weakness and tremors are common side effects and can progress to seizures

It can be normal for kittens to be somewhat tired and sore after vaccination, just like human children. Some may run a low-grade fever and not eat well for the first day or so after their shots. What is NOT normal would be any of the following symptoms: repeated vomiting, pale gums, difficulty breathing, or collapse/extreme weakness - these are signs of a serious allergic reaction. This is very rare following vaccination, but if it occurs it can be life-threatening - so if you observe any of these signs, make sure to get your kitten to the vet right away. If she is just tired and sore, it should be fine to monitor her at home - she should be feeling better within the next 24-48 hours.

Although Misty was improved when you brought her home and the tremors stopped, I would expect it will take her a few days to get back to normal. This is very common after a stressful event like a hospital stay. Give Misty 3-5 days to become reacclimated to her environment. Offer her lots of attention, treats, and keep stress as minimal as possible. If, after this period of time she’s still not acting normal, I would recommend taking her back in for a recheck examination.

This sign is very concerning and may eventually be in a life threatening condition. Fever or extreme discomfort may be present. I would strongly recommend an immediate check up to confirm this clinical suspicion and have the appropriate treatment performed. Antibiotics may be needed.

I am concerned as well. While a kitten can have some lethargy after receiving vaccines and deworming treatments, it's not normal for that to last more than a day or two. If she is eating and drinking normally, then you can wait until the morning to have her seen by a vet by a health check. Should she not be eating or drinking normally, then I would have her seen by an ER vet tonight.