Traumatic injury: This is the most likely diagnosis given the presentation of a wound with surrounding erythema. The uncertainty about how the wound occurred does not rule out a traumatic cause, as the patient might not recall or recognize the incident.

Insect bite or sting: This could cause a localized reaction leading to erythema and potentially a wound if scratched.
Cellulitis: An infection of the skin and subcutaneous tissues, which could present with erythema, warmth, and swelling, potentially with a wound if there was a break in the skin.
Ulcer (venous or arterial): Given the location on the calf, a venous ulcer is more likely, but an arterial ulcer could also present with a wound and surrounding erythema, especially if there's a history of vascular disease.

Necrotizing fasciitis: Although rare, this is a life-threatening condition that requires immediate attention. It can present with erythema, swelling, and pain out of proportion to the appearance of the wound.
Gas gangrene: Another severe infection that can cause tissue death and gas production in the tissues, presenting with severe pain, swelling, and potentially a foul odor.
Deep vein thrombosis (DVT): While DVT itself might not directly cause a wound, the associated inflammation and potential for ulceration or breakdown of the skin could lead to a wound, and it's crucial to consider in patients with risk factors.

Pyoderma gangrenosum: A rare skin condition that can cause painful ulcers, often without a clear cause, and can be associated with systemic diseases.
Ehlers-Danlos syndrome: A group of disorders that affect connective tissues supporting the skin, bones, blood vessels, and many other organs and tissues, potentially leading to easy bruising, poor wound healing, and other skin manifestations.
Malignancy (e.g., basal cell carcinoma, squamous cell carcinoma): Although less likely, certain types of skin cancer can initially present as non-healing wounds or ulcers with surrounding erythema.