I wouldn't waste your time going back to the pet store. They'll likely just exchange her, and she more than likely won't survive. This is one of the big reasons why we ALWAYS encourage people to break the cycle, and get pigs from a rescue who need and deserve a furever home. I know that isn't what you want to hear.
If she is on antibiotics, you SHOULD start to see an improvement in 3-4 days. But the antibiotics will likely upset her tummy so she may stop eating and if she does, you will need to hand feed her with critical care and a syringe. By handfeeding I mean force feeding. Guinea pigs are hardwired to always have food moving through their stomachs and if she doesn't eat you will lose her.
What antibiotic is she on? Is your vet an exotic specialist, or just a small animal vet?