Viral Conjunctivitis: This is the most likely diagnosis due to the presence of bilateral conjunctivitis and small white spots on the conjunctiva and buccal mucosa, which could be indicative of a viral infection such as adenovirus.

Allergic Conjunctivitis: This condition could also explain the bilateral conjunctivitis and erythema, especially if the patient has a history of allergies.
Bacterial Conjunctivitis: Although less likely than viral conjunctivitis given the description, bacterial conjunctivitis can cause similar symptoms and should be considered, especially if there is a history of exposure to someone with a similar infection.
Herpes Simplex Keratoconjunctivitis: This condition can cause conjunctivitis, erythema, and small white spots (dendritic ulcers) on the cornea, making it a plausible diagnosis.

Gonococcal Conjunctivitis: Although less common, gonococcal conjunctivitis is a serious condition that requires immediate treatment to prevent complications such as vision loss. It can present with severe conjunctivitis and should be considered, especially in sexually active individuals.
Chlamydial Conjunctivitis: Similar to gonococcal conjunctivitis, chlamydial conjunctivitis is a serious infection that can cause significant morbidity if not treated promptly.

Sjögren's Syndrome: An autoimmune disorder that can cause conjunctivitis and oral mucosal lesions, but it is less likely given the acute presentation.
Stevens-Johnson Syndrome: A severe condition that can cause conjunctivitis, oral mucosal lesions, and skin lesions, often in response to medications or infections. It is rare but should be considered due to its severity.
Behçet's Disease: A rare autoimmune disorder that can cause conjunctivitis, oral ulcers, and genital ulcers, among other symptoms. It is less likely but should be considered in the differential diagnosis due to its potential for serious complications.