Allergic Conjunctivitis: This is the most likely diagnosis given the symptoms of itchiness, swelling, and pain in one eye. Allergic conjunctivitis is a common condition that occurs when the eyes react to an allergen, leading to inflammation and the described symptoms.

Blepharitis: An inflammation of the eyelids that can cause itchiness, swelling, and pain due to bacterial or demodex mite infestation.
Dry Eye Syndrome: Although it primarily causes discomfort and vision problems, severe cases can lead to inflammation and pain.
Conjunctivitis (Viral or Bacterial): Infections of the conjunctiva can cause redness, swelling, and discharge, along with pain and itchiness.
Keratitis: Inflammation of the cornea that can result from infection or injury, leading to pain, redness, and sensitivity to light.

Acute Angle-Closure Glaucoma: A medical emergency that can cause severe eye pain, redness, and swelling, along with vision loss. It's crucial to diagnose promptly to prevent permanent vision loss.
Orbital Cellulitis: A bacterial infection of the tissues surrounding the eye that can cause swelling, pain, and redness. It requires immediate antibiotic treatment to prevent serious complications.
Endophthalmitis: A severe inflammation of the interior of the eye, usually caused by infection. It's a medical emergency that can lead to vision loss if not treated promptly.

Ocular Rosacea: A condition that can cause inflammation of the eyes, leading to symptoms similar to blepharitis, but is associated with rosacea of the skin.
Sjögren's Syndrome: An autoimmune disorder that can cause dry eyes and other systemic symptoms, though it's less likely to present with acute swelling and pain.
Trichiasis or Entropion: Conditions where eyelashes grow inward or the eyelid turns inward, respectively, causing irritation and potentially leading to the described symptoms, though these are more chronic conditions.