Cellulitis or Abscess: The description of the upper thigh back giving out, being very painful, lumpy, and the lumps being hard could indicate an infectious process such as cellulitis or an abscess. The hardness of the lumps might suggest an abscess, which is a collection of pus that has built up within the tissue of the body, indicating a localized infection.

Lipoma: Hard lumps in the thigh could be lipomas, which are benign tumors made up of fat tissue. They are usually painless but can cause discomfort if they press on nearby nerves or develop in an area that restricts their growth.
Fibromatosis: This condition involves the growth of fibrous tissue, which can form hard lumps. It can occur in various parts of the body, including the thigh.
Hematoma: A collection of blood outside of blood cells, which could be caused by trauma to the area, leading to pain and swelling.

Deep Vein Thrombosis (DVT): Although DVT typically presents with swelling, warmth, and discoloration of the leg, it's crucial not to miss this diagnosis due to its potential for leading to pulmonary embolism, a life-threatening condition.
Soft Tissue Sarcoma: While rare, sarcomas can present as hard lumps in the soft tissues, including the thigh. Early detection is critical for treatment outcomes.
Osteosarcoma: A type of bone cancer that can cause pain and swelling in the affected limb. It's less likely but critical not to miss due to its severity.

Panniculitis: Inflammation of the subcutaneous fat, which can cause hard lumps and pain. It's less common and can be associated with various conditions, including autoimmune diseases.
Nodular Fasciitis: A rare, benign condition characterized by the growth of nodules in the fascia, the tissue that surrounds muscles, which can mimic more serious conditions.
Eosinophilic Fasciitis: A rare condition that involves inflammation of the fascia, leading to thickening and hardening of the skin and underlying tissue, which can cause pain and limited mobility.