A painful and swollen bag on the top of the foot is considered to be related to trauma, tendon sheath cyst, infection, lipoma and so on. 1. Trauma: It may be caused by localized bleeding after a broken foot, after which localized swelling on the top of the foot with bruising may occur. 2. Tendon sheath cysts: tendon sheath cysts are benign cysts near the joints that may be related to chronic injuries and connective tissue mucous degeneration, and may appear as lumps on the surface of the dorsal part of the foot with pain and soreness. 3. Infection: when the skin on the surface of the foot is infected by bacteria such as Staphylococcus aureus and involves the deep tissues, it can lead to lumps on the surface of the foot, accompanied by redness, swelling, fever, pain and other symptoms on the surface of the skin. 4. Lipoma: Lipoma is a kind of common soft tissue benign tumor, which is the tumor of normal fat-like tissues, and the main clinical manifestation is the lump, when the lipoma is bigger, the compression of local tissues can also cause pain. There are many reasons for the pain and swelling of a bag on the top of the foot, and it is necessary to carry out a detailed examination to make a clear diagnosis and carry out the corresponding treatment after determining the cause of the disease, so as not to cause adverse effects.