I am sorry to hear you are worried about Scarlett's back pain. My name is Rebecca. I am a veterinarian and will do my best to help.
If we give aspirin to cats, the dose is usually half of an 81 mg pill, and that can only be given once every 3 to 4 days, no more often than that. If she did get a whole 81 mg pill, you may see some side effects such as vomiting, and possibly internal bleeding from the blood thinning properties of aspirin. It can also cause kidney damage, so if she already has kidney problems she should probably not take any more aspirin.
If she is eating, not vomiting, and if her gums appear a normal pink color, that is good. If she stops eating, if she vomits, or if her gums appear pale I would have her checked by your veterinarian.
If you want a liquid medication for pain, I would look into getting a prescription pain medication called buprenorphine. It would be safer in a cat her age, with kidney problems.
Let me know how she is doing and what else I can help with or answer. Rebecca