I would definitely suggest seeing an exotics vet/guinea pig vet asap as most normal vets are not experienced enough to deal with Guineas and sometimes miss vital signs.
I had a very similar situation just last month with one of my boys. He had lost a great deal of weight, was hiding away all the time and although he was trying to eat, whenever he picked up a piece of hay or veggie he would open his mouth as if he were choking. We ended up having to take him to the clinic a number of times within the space of a week. Our exotics vet took a look inside his mouth and ended up performing an x-ray - it turned out he had a dental problem that was preventing him from eating. Several weeks after his operation he’s back to eating well and seems to be his normal self.
Critical care may be a good idea to try and can be bought from most pet shops. It’s a paste that you can either give in a bowl/spoon if your Guinea will willingly eat it or or it can be syringe fed slowly every few hours. It’ll help to at least keep your piggy going until you can get her to a vet.
I hope everything will be okay!