You are right, it could be that the dress your daughter wore was not washed for a long tine and kept in the attic.
Please give her any anti allergic medication and apply calamine
lotion over the rashes.
If it is getting severe and spreading to other parts of the body then a visit to ER and a steroid
shot may be given by your doctor.
Wash any old dresses in detergent or at the dry cleaners before wearing them.
Hope this helps,
Dr Anjana Rao