Hardening skin around a wound can mean three different things, and they point in different directions.
As a sign of infection. Increased firmness around the wound is one of the signs of infection for a diabetic foot ulcer, along with redness, increased warmth or swelling, extra drainage, pus, odor, fever or chills, and increased pain.
As a sign of cellulitis. A tight, glossy, stretched appearance of the skin is one of the symptoms of this skin infection, together with warm skin in the area of redness and redness that gets bigger as the infection spreads.
As a sign of blood pooling. With venous insufficiency, dark red, purple, brown, hardened skin is a sign that blood is pooling. That is one of the early signs that appear before an ulcer forms, and it tends to develop over months rather than days.
So the timescale is what separates them. Skin that has hardened gradually on a lower leg, with discoloration, points to venous insufficiency. Skin that has become firm, tight or glossy around a wound over days points to infection.
One more thing can harden skin in a way nobody wants. Pressure sores can be covered in dead skin that is yellow, tan, green, or brown. And eschar — a dry, hard covering — is in some circumstances deliberately left in place.
This page cannot tell you which of these your skin is doing. What it can do is give you the words, and the one question worth answering before you call: has this come on over days, or over months?