How to solve spandex from causing rash on back upper thigh?

SPANDEX RASH: If you are developing a rash from spandex, do not ever wear it again! you may be allergic to it. Get a different type of material.

Cause of rash 1st?: You must first determine if it is due to a sensitivity to a component of the spandex i.e., latex allergy/sensitivity or due to chaffing for the shear forces of the garment on the skin. In the former you must discontinue and consider topical antihistamines/corticosteroids. In the latter, consider powders or lotion.