There are a few things which can cause bald patches. They all need a vet to diagnose and treat.
We can only speculate as we cannot see your piggy, nor can we diagnose in any event, but one thing you think of in a new piggy is potential ringworm.
If your vet does diagnose ringworm then you require stringent hygiene and the correct treatment as it is highly contagious to other animals and humans.
Does she live with another piggy? If she does have a companion and it does turn out to be ringworm, then the companion would also need treating. Treating the companion piggy during the 10-14 incubation period can prevent the companion from ever showing symptoms.
If she does not have a companion please do ensure you look to get her a companion as soon as she is treated for any potential issue. Piggies are highly social and always need to be kept in pairs. Particularly a piggy of her age should not be alone.
The guide below details ringworm and how to deal with it if that is what a vet diagnoses her with.
New Guinea Pig Problems: Sexing & Pregnancy; URI, Ringworm & Parasites; Vet Checks & Customer Rights
Ringworm: Hygiene, Care And Pictures
The diet should consist of unlimited amounts of hay - it needs to make up the main proportion of what she eats in a day. She can have one cup of leafy veg per day but any new veg needs to be introduced slowly so as to not cause digestive upsets. Do not give fruit daily as it is too high in sugar - fruit should not be in the diet at all but if you are going to give some it should be one very tiny amount as a very occasional treat and no more than once a week.
She can also have one tablespoon of plain pellets. The pellets are the least healthy proportion of the diet and should always be kept very limited. It may say complete nutrition but it does not form the main part of the diet.
Long Term Balanced General And Special Needs Guinea Pig Diets
New Owners' Essential Information and Practical Tips Starter Collection