My 11 year old sister as a red rash on both her lower arms?

My 11 year old sister as a red rash on both her lower arms and she said the itch like hell and shes been sitting on my bed for the last 4 hours and have not touched anything what can cause this I am lost ? and don’t know what to do and its happend before what do i do ? shes already had the measles please help me out
what would she me allergic too ?

she may be allergic to something then