Corneal abrasion or superficial foreign body: The patient's complaint of a bee sting in the eye, despite having 20/20 vision and no swelling or redness, suggests a possible minor injury to the cornea. A corneal abrasion or a superficial foreign body could cause discomfort without significantly affecting vision or causing noticeable external signs.

Conjunctival or corneal foreign body: Even if the vision is unaffected, a small foreign body (like a bee stinger fragment) could be lodged in the conjunctiva or cornea, causing irritation but not necessarily impairing vision or causing significant redness or swelling immediately.
Allergic reaction (mild): An allergic reaction to the bee sting could be mild and not immediately apparent, especially if the patient has not had a previous severe reaction. Symptoms might be delayed or subtle.

Intraocular foreign body: Although less likely given the lack of immediate symptoms, an intraocular foreign body (if the stinger penetrated the eye) is a critical diagnosis not to miss due to the potential for severe vision loss or infection.
Corneal ulcer or infection: Delayed onset of infection or a corneal ulcer could occur if the eye was indeed penetrated by the stinger or if there was a secondary infection, which would require prompt treatment to preserve vision.
Anaphylaxis (delayed onset): While the patient does not currently show signs of anaphylaxis, this condition can sometimes have a delayed onset, and missing it could be fatal.

Endophthalmitis: A rare but serious infection of the inner eye that could result from a penetrating injury, such as a bee stinger entering the eye. It would likely present with severe symptoms, including vision loss and eye pain, but could be considered in the differential due to its severity.
Sympathetic ophthalmia: An extremely rare, bilateral granulomatous uveitis that can occur after a penetrating eye injury. It's a remote possibility but one that could have significant implications for the patient's vision in both eyes.