I would not be surprised if her arthritis is getting to her "good" muscles and joints. All of her "better" bits have been compensating for her weaker bits, and it is probably causing excess wear and tear. Upping or changing her pain meds is what I woiuld probably do, too. My CH baby is only 2 1/2 years old; and, fortunately, hers is mild.
Since Amy doesn't like heating pads, you might try bringing her into the bathroom while you shower. The moist heat would probably help her to feel better. Purrs and well wishes to Amy and your joints


PS. Surprise We though our CH baby Phoebe was fixed. That is, until she came into heat for the first time at 2 1/2 years old Vet visit in near future