Without other accompanying symptoms like pain, temperature, the cause for the rash is rarely something serious.
This rash can be due to skin disease or irritation from detergent, soap, or fabric.
A rash that begins with a single patch on one leg, but soon multiple lesions appear on both lower legs and may later affect the trunk and arms, could be an eczema or dermatitis, since most often, a rash affecting the lower legs is a type of dermatitis.
See your physician for referral for dermatologist!