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.