Daughter 6 yo gets red splotches all over her left leg when being very active not itchy/raised only left, goes away after resting?

Cholinergic Hives: Cholinergic urticaria is a medical term used to describe a subcategory of physical urticaria (hives). It is characterized by a hypersensitive response in the skin as a result of the body increasing in temperature (passively or actively), or the precipitating release of sweat. Some common treatments include taking antihistamines (such a allegra, (fexofenadine) zyrtec, or stronger ones), avoiding known triggers.

May be allergy : You can develop an allergic response during exercise but would typically involve whole body as would a heat rash. This is a situation in which " a picture is worth a thousand words" try and precipitate the rash just prior to a doctor's visit or get a good picture for them to review along with more detailed history.