Lipoma: A benign tumor composed of fat tissue, which can appear suddenly under the skin, including on the lower leg. It is usually soft, movable, and painless.

Cyst: Such as an epidermoid cyst or sebaceous cyst, which can form under the skin and appear as a sudden growth. These are usually benign and can be movable.
Abscess: A collection of pus that has built up within the tissue of the body, indicating a bacterial infection. It can appear as a sudden, painful swelling under the skin.
Hematoma: A collection of blood outside of blood cells, which can occur due to injury and appear as a sudden growth under the skin.

Deep Vein Thrombosis (DVT): Although not typically described as a "growth," DVT can cause sudden swelling in the leg, which might be mistaken for a growth under the skin. It's a blood clot that forms in a vein deep inside your body and can be life-threatening if it breaks loose and travels to your lungs.
Soft Tissue Sarcoma: A rare type of cancer that can appear as a lump or swelling under the skin. It's crucial to diagnose early for effective treatment.
Infected Bursae: Bursae are fluid-filled sacs that cushion bones, tendons, and muscles near joints. An infected bursa can cause swelling and pain and might be mistaken for a growth.

Dermatofibrosarcoma protuberans (DFSP): A rare type of skin cancer that can appear as a hard lump under the skin, often on the torso but can occur on the lower leg.
Panniculitis: Inflammation of the subcutaneous fat, which can cause sudden swelling and might appear as a growth under the skin.
Lymphoma: Although rare, lymphoma can manifest in the skin and appear as sudden growths or swellings, including on the lower leg.