It is always hard to tell what is wrong even when you are looking at a picture. My first guess is always mange mites (you can´t see the eggs or the mites, they are too small) and a recommendation to treat with ivermectin. A vet can do this accurately. If this pig has never been treated with ivermectin, it´s a good idea anyway, if she is at least 12 ounces in weight. You can read about hairloss, mites and parasites if you click on the Guinea Lynx button above and check the index for Hair Loss. If it was mites, the cavy might have pulled the hair out and bit at it as they are very painful.
Fungus is the second possibility. Washing the area with Nizoral shampoo or following some of the other suggestions on Guinea Lynx may help.
I still always think mites first, fungus second. If you find a qualified exotics vet who has seen alot of cavies, he/she may be able to do a better diagnosis.