We change her water everyday and make sure she has a clean house and a good diet. Guinea pigs are prone to vitamin deficiencies, mange mites and allergic dermatitis. Be sure to use guinea pig food, not rabbit pellets which look similar. Guinea pigs require more Vitamin C and E. Excessive crusting around the eyes often indicates a Vitamin C deficiency. You can supplement with citrus or Vitamin C fortified beverages.
Mange mites cause a rash and hair loss, usually worse on the back and rump.
Allergic dermatitis is often caused by pine shavings, and is worse on the chest and abdomen. You can use human baby shampoo to bathe the pigs, and a cream rinse is a good idea.