Red spots on the thighs may be caused by atopic dermatitis, insect bite dermatitis, or papular urticaria. Because the skin of the area itself is prone to allergies, there may be localized red spots or large patches of redness, sometimes accompanied by varying degrees of itching. Of course, the red spots may also be purpura, which may be due to allergies or infections, or blood-related factors, resulting in localized subepidermal bleeding. So if it is purpura, it may be accompanied by varying degrees of joint pain or general discomfort, but there is usually no itching.