Please have her vet checked by good vet during regular hours tomorrow! All we can do is guess as much as you. Back leg paralysis in older guinea pigs is not uncommon, but it can have different causes (most commonly sudden drop of calcium or arthritis, neurological problems, internal swellings/lumps, problems in the urinary tract or blood clots etc.) The good news is that in most cases, mobility is eventually regained, but treatment depends on your vet's findings.
Make sure that she has hay (which makes 80% of the daily food intake) close by, as well as water. Weigh her daily at the same time in the feeding cycle and step in with syringe feed top up as soon as she loses more than 50g. Our syringe feeding guide also contains tips what you can do in an emergency with what you have got at home and where to source our stuff.
Complete Syringe Feeding Guide
You may also want to give your girl a gentle bum bath with hand warm water to remove all those poos. Change her bedding in the sleeping area where she is more twice daily to prevent urine scald.
When you see a vet, especially as an emergency, please make sure that you are not fobbed off with a steriod injection. In rodents, steroids slow down organs, instead of pepping them up.
The Problems With Steroids And Why They Shouldn't Be Used.