Whats wrong with my guinea pig?

my guinea pig has been seen by two vets now and is still in the vets, she has lost interest in food but only now taking food watered down by syringe, the vets have both checked her teeth and backside and there is no obvious signs on what is wrong with her but she is walking strange and seems to be walking on her heels the vets dont know why but when is have just given her her favourite gr she ate some and then put her head on the floor of the cage and didnt seem interested in us but doest look ill ie; coat or eyes