Pilonidal Cyst or Abscess: This condition is a common cause of tender areas on the buttocks, especially with a history of fistula. Pilonidal cysts can become infected, leading to abscess formation, which may drain and form a fistula.

Furuncle or Carbuncle: These are infections of the hair follicles that can cause tender, swollen areas on the buttocks. If they become severe, they can lead to abscess formation and potentially fistulae.
Hidradenitis Suppurativa: A chronic skin condition characterized by recurrent, painful abscesses and nodules, often in areas with apocrine glands like the buttocks. It can lead to fistula formation.
Perianal Abscess or Fistula: Although more common around the anus, perianal abscesses and fistulas can extend to the buttocks, especially if the infection spreads.

Necrotizing Fasciitis: A severe, life-threatening infection of the skin and subcutaneous tissue that requires immediate medical attention. It can present with pain out of proportion to the visible skin findings.
Osteomyelitis: An infection of the bone that can cause pain and tenderness over the affected bone. In the context of a fistula, it could be related to an underlying bone infection.
Tuberculosis (TB) of the Skin: Although rare, cutaneous TB can present with a variety of skin lesions, including abscesses and fistulas, especially in immunocompromised patients.

Actinomycosis: A rare bacterial infection that can cause abscesses, fistulas, and sinus tracts. It's more common in the face and neck but can occur in the buttocks.
Crohn's Disease with Perianal Involvement: While Crohn's disease is more commonly associated with gastrointestinal symptoms, it can cause perianal fistulas and abscesses, which might extend to the buttocks.
Foreign Body Reaction: In rare cases, a foreign body (like a needle or other object) can cause a localized reaction, leading to abscess formation and potentially a fistula if it migrates through tissues.