Perhaps it is a systemic candida (yeast) growth? If so, then it is inside as well as out. It would fade periodically if she suddenly eats a bunch of yogurt or takes acidophilus, say, for a cold, or just happens not to eat a lot of the foods that contribute to candida overgrowth, but she'd have to do a lot more than that to make it actually go away, like a full-on candida cleanse (some companies provide the herbal components to this cleanse; check with your local natural market). While doing this cleanse, your daughter would also have to make a dietary effort. I've never done it, but I do know that cutting out all added sugar is important. No alcohol, and no caffeine, either, probably, and certainly no dairy products (candida thrive on dairy). I'm not sure about the rest of the guidelines for doing a candida cleanse. Let's do a quick check online:
The following link includes symptoms of candida overgrowth.
"Most commonly, patients affected by candida will exhibit constipation, abdominal pain, headaches, numbness, fatigue, skin rashes, persistent fungal infections (jock itch, vaginal yeast infections, bladder infections, nail fungus or athlete's foot) and mood changes."
It also says, "Factors which may aggravate candida and contribute to its overgrowth are long-term use of certain medications including antacids, antibiotics and oral contraceptives; food allergies; smoking; hormone fluctuations such as those occurring with monthly cycles or pregnancy; diets high in sugar or diabetes." Your daughter might be aware of some factor/s that make sense in relation to her rash and any other of the symptoms listed above that she might be experiencing.
http://www.ehow.com/how_###-###-####_do-candida-cleansing...
The author cautions that it is not advisable to do a candida cleanse without first having one's doctor check for candida overgrowth.
I hope this helps.
If it's not candida, maybe it's an allergic reaction to something in her environment, like latex, or to chemical cleaners in the house or her workplace.
L.