Wow 20 years old, that is wonderful.
Your cat absolutely needs to go in for bloodwork. My first guesses are:
Diabetes or Thyroid issues, which are both easily controlled by meds.
You may also check to see if there are any foundations that assist low income people with help with vet bills, or a vet that will accept a payment plan. Your local humane society or SPCA may have those answers readily available for you.
Good luck and please keep us updated.
Cat maid to:
Puddles, female Himalayan (16 yrs)
Jasper, male Ragdoll ?? (approx 9 yrs)
Sweet Pea, female silver tabby DSH (approx 7 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 6 yrs)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey