Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your little one has been very lethargic after her veterinary appointment and vaccines, including her Rabies vaccine, yesterday.
I am very glad to hear that she is eating and drinking normally.
It is possible that she is having a mild reaction to the vaccines. These sorts of reactions can cause a low grade fever, pain at the injection site, make her feel itchy, especially her paws and face, and can sometimes lead to vomiting, diarrhea, facial swelling and hives.
If she seems especially uncomfortable or itchy I recommend giving her an antihistamine to control allergic symptoms for at least 72 hours. You can give Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to dogs) at a dose of 1mg to 2mg per pound of body weight every 8-12 hours. That's 1/2 of 25mg tablet every 8-12 hours for a kitty that is 8-15 pounds.
Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.
She should be much more comfortable in about 2 to 3 hours.
If she seems truly miserable, febrile and achey, you can give her one dose of one half of a baby aspirin (81mg size). Do not repeat this as it can be hard on a cat's gastrointestinal system and cats have a hard time metabolizing aspirin. Do not give if she is vomiting or not eating anything.
If she begins vomiting or has diarrhea, has any difficulty breathing or facial swelling it is time to have her seen on an emergency basis for a steroid injection.
You should report this reaction to her veterinarian. It should be noted on her permanent record and the manufacturer should be informed.
In the future she should only be given one vaccine at a time and they may wish to pretreat her with anti-histamines or steroids when she is vaccinated as a precaution to prevent such reactions and/or send her home with some anti-inflammatory medications.
This is really not something to be overly concerned about, but we do want to keep a close eye, and make sure that she is as comfortable as possible.
Please let me know if you have any further questions.