Pilonidal Cyst: This is the most likely diagnosis given the location (tailbone), symptoms (pain, pressure), and history (previous drainages, recurrence). The patient's report of her boyfriend pulling two hairs out of the area also supports this diagnosis, as pilonidal cysts are often associated with ingrown hairs.

Furuncle or Abscess: The patient's symptoms of pain and pressure, along with a history of previous drainages, could also suggest a furuncle (boil) or abscess, especially if the pilonidal cyst has become infected.
Epidermoid Cyst: Although less common in this location, an epidermoid cyst could present similarly, with a painful, fluctuant mass, especially if it becomes infected.
Infected Sebaceous Cyst: This could also present with similar symptoms, especially if the cyst becomes infected, leading to pain, redness, and swelling.

Malignant Transformation: Although rare, it's crucial not to miss the possibility of a malignant transformation in a long-standing cyst, especially if there are changes in symptoms or appearance.
Osteomyelitis: Infection of the bone could occur, especially if the cyst or abscess erodes into the bone, leading to severe and potentially life-threatening complications.
Necrotizing Fasciitis: This is a rare but life-threatening condition that could occur if the infection spreads to the fascia, requiring immediate medical attention.

Teratoma: A type of germ cell tumor that could present as a cystic mass, although this would be extremely rare in this location and context.
Hydatid Cyst: Caused by a parasitic infection, this could present as a cystic lesion, but it would be uncommon in this context and location, especially without a history of exposure to the parasite.
Gluteal Hidradenitis Suppurativa: A chronic skin condition that could lead to painful nodules and abscesses in the gluteal area, but it typically involves more than one lesion and has a different characteristic appearance.