Hi, i have a small red rash appeared three days before on my left thigh, after then, it started spreading to my right leg and thighs, what could it be?

Rash on Thigh: A rash on your thigh can have a variety of causes, and it may indicate something occurring around the thigh itself or suggest a systemic (body-wide) condition. Causes include: contact dermatitis, allergies, eczema, psoriasis, infections like yeast or bacteria, chaffing of the skin, medications, detergents, lotions and others.

Many possibilities: This rash would likely need to be examined and more history obtained to be able to provide you with useful input about the diagnosis or possible treatment.