Local Reaction to Vaccine: This is the most common cause of a lump in the arm after receiving the Gardasil vaccine. The vaccine, like many others, can cause local reactions such as redness, swelling, and induration at the injection site, which may be perceived as a lump.

Granuloma: A granuloma can form as a reaction to the vaccine or its components, leading to a lump at the injection site. This is less common than a simple local reaction but still within the realm of expected side effects.
Abscess or Infection: Though rare, any injection can potentially introduce bacteria into the skin, leading to an abscess or infection. Symptoms would typically include increased redness, warmth, swelling, and possibly pus.
Lipoma or Other Benign Tumors: Pre-existing lipomas or other benign tumors might become more noticeable after the injection due to inflammation or swelling in the area.

Necrotizing Fasciitis: A severe and potentially life-threatening infection that requires immediate medical attention. It's extremely rare as a complication of vaccination but would be catastrophic if missed.
Deep Vein Thrombosis (DVT): Though very unlikely, a DVT could present as a lump or swelling in the arm. Given the potential for severe consequences, such as pulmonary embolism, it's crucial not to miss this diagnosis.

Dermatofibrosarcoma Protuberans: A rare type of skin cancer that could potentially be mistaken for a lump at an injection site, though it's not directly related to the vaccine.
Foreign Body Reaction: In rare cases, a reaction to the vaccine adjuvants or other components could lead to a foreign body reaction, which might present as a lump.
Panniculitis: An inflammation of the subcutaneous fat that could be triggered by the vaccine, presenting as a lump or swelling in the arm.