Hello and thanks for researching this very important question!
Most GPs are bald behind the ears, so this alone does not indicate a serious problem.
Guinea pigs can develop a variety of skin problems that include:
1) Ringworm Fungus--the most common cause of round spots of hair loss all over the body that can be itchy and scaly
2) Mange--a tiny skin parasite that causes a generalized dermatitis (skin inflammation), also known to cause enough pain to bring on seizures!
3) Scurvy--a dietary insufficiency helped by vitamin C supplementation...potency of pet store brands varies, so I like to use a pediatric liquid found at the regular pharmacy...an average-sized adult should receive 15-20 mg daily directly by mouth.
4) Self-mutilation ("barbering")...account for hair chewed short on the rump and hind legs.
5) Allergy to bedding (cedar and now even pine are known to cause skin allergy symptoms--you may need to experiment with different recycled paper products in future)
6) Hormonal conditions...including hair loss associated with post-weaning nutritional deficiency and certain kinds of ovarian and adrenal gland cysts.
7) Stress...changing house can certainly cause some more-than-usual hair loss, but this should regrow normally.
Here is a link to a site that gives good information about diagnosing and treating skin and hair problems in GPs:
This site has photos of the various conditions that are very large and typical of the illnesses in question. This is also information that you can share with a friendly vet in case you feel you need to have a prescription ordered for you.
Not all medications are safe to use in GPs, so do stick to the recommendations listed.
Ringworm and mange would be transmissible to humans, so contact your personal doctor if you have questions about your health. Keep the enclosure super clean, and even use a dilute bleach solution (3oz to 1 gallon of water) to rinse the tank with every bedding change until you are sure that these two parasites are controlled.
Here are some sites that advertise vet-finder indexes:
***You might try experimenting with softer brushes (like those used for infants) in case she is just not used to the sensation and the bristles are too rough for her skin.***
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