Hi! have you been seeing one of their piggy savvy vets?
Back leg paralysis can have many different causes. If it is just one leg, you are not looking at the most common one in older guinea pigs, a sudden drop in calcium, as that affects both hind legs equally.
Has your vet checked for a) injury/sprain to one of the joints, b) arthritis, c) a swelling or build-up of fluid (a soreness/swelling in the adrenal gland area caused temporary loss of movement in one of the back legs of my 8 year old Nerys; it led to a build-up of fluid in the following weeks before disappearing as mysteriously and suddenly as it had come on), d) a neurological problem, e) a small blood clot in one of the leg vessels.
As long as your piggy still has the will to live and eat and is not obviously in pain, she still has quality of life, but you may need to adapt things around her. Change the bedding or cosies 1-2 times daily where is she is sitting most of the time. Make sure that water and hay (which makes up to 80% of the daily food intake) are very close by and can be directly accessed from her lair.
Weigh daily at the same time to monitor the food intake and if necessary top up with syringe feed or mushed pellets. I let Nerys eat from a spoon to make sure that it always was her decision and her will to live that was the decider, and not my just care that kept her alive.
Check all her legs daily; guinea pigs with limited mobility are more prone to develop sores on the pads of their feet, especially the weight bearing front paws (bumblefoot). They are also often developing urine scald from sitting in their waste (loss of hair in the genitalia area and then sore skin).
Please also consider that metacam is not just a painkiller, but also an anti-inflammatory.