What are these little red welts on my legs that don't itch, or hurt, but they appeared over night?

Urticaria. When you have an allergic reaction to a substance, your body releases histamine and other chemicals into your bloodstream. This causes itching, swelling, and other symptoms. Hives(urticaria) are a common reaction, especially in people with other allergies such as hay fever. When swelling or welts occur around the face, especially the lips and eyes, it is called angioedema.