Hi, definitely do not give her the acetaminophen, that is extremely toxic to cats.
How much fish oil are you giving her?
Cats are very sensitive to the effect of NSAID(non steroidal anti inflammatory) then dogs and humans. There are not a lot of NSAID approved for use in cats.
NASAID can cause kidney/liver and gastro intestinal ulcers.
I would stop at once the ibuprofen, and see your veterinarian. You may have already caused some kidney damage. They may put your kitty on gastric protectants.
There are no safe anti inflammatory at the human pharmacy that are safe to use in cats.
Your kitty may have arthritis, or maybe another disease process requiring other type of medication, or treatment approach.
It is better to see your primary care veterinarian, to first check that you have not caused any problem with ibuprofen, make sure she really has arthritis, and discuss your treatment options if this is the case: meloxicam intermittently, adequan injections, fish oil, glucosamine supplements, acupuncture, physical therapy, other.
Let me know if you have more questions, or if you need additinal information.
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