Importance of hay in diet

Question
My guinea pig is 2 yrs. old. We have always included timothy hay as part of his daily diet. Two days ago we noticed that he wasn't feeling well and he had a swelling in his right cheek.  We brought him to our vet (that we use for our dog).  She said that he probably got a cut inside his mouth from the hay. She gave him an antibiotic injection and he'll be on oral antibiotics for a week.  She told me to completely remove hay from his diet.  This confuses me because I thought that hay was important for guinea pig health.  (I don't think this vet has much experience with guinea pigs because when I made the appointment, the receptionist had to check to find out if they see guinea pigs.)  What do you think?
Thanks

Answer
I think this vet is totally clueless when it comes to exotics. Guinea Pigs are "exotics" and must ONLY be seen by experienced exotic vets. Dog and cat vets learned nothing about cavies when they went to school.

Timothy hay is essential in their diet, they cannot go without it. Timothy hay is far too soft to have injured his mouth. It sounds as though he could have a mouth sore from too much fruit, or perhaps he has something wrong with his teeth.