Blepharitis or Conjunctivitis: Given the recent history of stye excision and the use of antibiotic eye drops, a recurrence or persistence of infection is possible. The symptoms of unilateral injection, pain, and photophobia are consistent with these conditions.

Dry Eye Syndrome: This condition can cause eye pain, photophobia, and injection, especially if the patient has an underlying condition or is experiencing environmental factors that exacerbate dryness.
Corneal Abrasion or Ulcer: The recent surgery could have led to a corneal injury, and the symptoms are consistent with such an injury. The finished antibiotic course might not have been sufficient to prevent a secondary infection.
Iritis or Uveitis: Inflammation within the eye can cause pain, photophobia, and injection. The recent surgical intervention could be a precipitating factor.

Endophthalmitis: Although less likely given the finished antibiotic course, this is a potentially devastating infection that can occur post-surgically. It requires immediate attention and treatment.
Acute Angle-Closure Glaucoma: This condition is an emergency and can present with severe eye pain, photophobia, and injection. It is crucial to rule out this diagnosis due to its potential for permanent vision loss.
Orbital Cellulitis: An infection of the tissues surrounding the eye, which can be a complication of recent surgery or a stye. It is serious and requires prompt treatment.

Scleritis: Inflammation of the sclera, which can cause severe pain, photophobia, and injection. It is less common but should be considered in the differential diagnosis.
Keratitis: Inflammation of the cornea, which could be due to various causes including infection or an immune response. The symptoms can overlap with those presented.