Hello
Thanks for posting your query .
Though your query is too short and lacks details, I will try to help you out.

I assume that she has no other positive details like history of fever, any medication or drug ingestion, past history of allergy etc.
If yes please mention the detailed history.
If this red rash is the only problem which has developed suddenly overnight then it is most likely either an allergy to any food, contact withchemical, clothing etc. or a skin infection viral more probable than bacterial.
Since any skin lesion needs thorough examination and investigation for exact diagnosis and treatment, it is advisable that she consult a dermatologist if the rash persists for more than a day If there is development of any symptom like fever, vomiting, diarrhoea or breathing difficulty or there is bleeding or pus like discharge from skin then seek prompt consultation.
If there is mild itching , burning only and no new lesions then applying topical lotion like calamine for soothing effect and oral antihistamines like cetrizine or fexofenadine may help to resolve the rash.
If it is an infection then it wil not resolve and she may develop other signs and symptoms, so its best to visit your doctor for appropriate antibiotics.
Hope I have answered your query.
Ill be available for your follow up queries.
Regards

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