Seroma: This is a common complication after breast reduction surgery, where fluid accumulates at the surgical site, leading to swelling and pain. The timing of 9 weeks post-surgery is plausible for a seroma to develop or become symptomatic.

Hematoma: Although less common at 9 weeks post-op, a hematoma could still occur, especially if there was a recent trauma or anticoagulant use, leading to swelling and pain.
Infection: Surgical site infections can occur weeks after surgery, presenting with swelling, pain, redness, and warmth. The risk factors and clinical presentation would guide this diagnosis.
Fat Necrosis: This is a known complication of breast reduction surgery, where fatty tissue dies due to inadequate blood supply, leading to firm masses, swelling, and sometimes pain.

Breast Cancer: Although extremely rare as a direct complication of breast reduction, any new mass or persistent symptoms in the breast warrant investigation to rule out malignancy.
Necrotizing Fasciitis: A severe and life-threatening infection that requires immediate recognition and treatment. While rare, the consequences of missing this diagnosis are severe.

Surgical Site Abscess: A localized collection of pus that could occur at the surgical site, more likely in the early post-operative period but possible at any time, especially with signs of infection.
Lymphedema: Swelling of the arm or breast on the side of the surgery due to disruption of lymphatic drainage, which is more common after lymph node removal but can occur after any surgery that affects lymphatic pathways.
Foreign Body Reaction: A reaction to sutures or other materials used in surgery, which could potentially cause swelling and pain, although this is less common with modern surgical materials.