Whats wrong with my guinea pig?

shes 7 years and shes been scratching and bitting her hair off and the lower part of her body is almost bald. we took her to the vet cause we thought she had mites, they did a skin scraping and theres was nothing on her skin. we didnt do a blood test cuz the vet said it would be painfull for her and would increase stress. so he told us to put vitamin C tablets in the water cause he thought maybe she had vitamin C deffeincy. but theres absolutely no improvement. her sickness i know is not contagious cause i have 2 others and there perfectly fine. shes also lost weight but shes preety mobile for her age. when i look at her she looks like zoned out and unfocused her waste is normal and she eats enough, we give her veggies and she always has timothy hay. i have no idea whats wrong with her