Alright, weight loss in Guinea pigs indicate some serious problem or disease. And hair loss indicates stress, some sort of illness and even loneliness. One can't tell what is wrong with your pig by just knowing these two symptoms but yes it's high time to get it checked by a vet. When pigs are ill they stop eating and that is why they lose weight. And when it won't eat food then the deficiencies will make its hair fall. If she is not eating and make her eat. And take her to vet as soon as you can.
My guinea pig has gone very thin,she is eating normal and her coat is very shiny,I don't know how old she is because she was a rescue about two years ago,she lives in a very large container in my hall which is big and warm,I give her all the best food from my vets and plenty of fresh veg and hay,I hope you can help me.