Contact Dermatitis: This is a common condition that can cause erythema (redness) on the skin after contact with an irritant or allergen. The fact that the area is improving suggests a self-limiting condition like contact dermatitis, which often resolves once the offending agent is removed.

Eczema (Atopic Dermatitis): A chronic condition characterized by itchy, inflamed skin. While it can cause erythema, the absence of itching and the improvement without specific treatment make it less likely but still possible.
Insect Bite Reaction: Reactions to insect bites can cause localized erythema and can improve without treatment, fitting the description provided.
Folliculitis: An inflammation of the hair follicles, which can cause redness and sometimes improves on its own, although it might be accompanied by some pain or swelling, which are not present in this case.

Deep Vein Thrombosis (DVT): Although the patient reports no swelling or pain, DVT can sometimes present with minimal symptoms, especially in its early stages. The erythema could be a sign of inflammation associated with a DVT, making it crucial not to miss this potentially life-threatening condition.
Cellulitis: A bacterial skin infection that can cause erythema, swelling, and pain. While the patient reports no swelling or pain, early stages of cellulitis might not have all these symptoms, and it's essential to consider this diagnosis due to its potential for serious complications if not treated.

Erythema Nodosum: A condition characterized by the sudden formation of painful nodules under the skin, typically on the lower legs. While it can cause erythema, the absence of pain or nodules makes this less likely.
Vasculitis: Inflammation of the blood vessels, which can cause a variety of skin symptoms including erythema. However, vasculitis often presents with more systemic symptoms or a more complex rash, making it a less likely but not impossible diagnosis in this scenario.