Recurrent Aphthous Stomatitis (RAS): This condition is characterized by the periodic appearance of benign, painful ulcers (canker sores) in the oral cavity, often accompanied by fever, especially if the ulcers become secondarily infected. It's a common condition that matches the symptoms described.

Herpes Simplex Virus (HSV) Infection: HSV can cause oral ulcers that resemble canker sores, and primary infections are often accompanied by fever. The presence of multiple small vesicles that rupture to form ulcers is a hallmark.
Hand, Foot, and Mouth Disease (HFMD): Caused by viruses like Coxsackievirus A, HFMD can present with oral ulcers and fever, along with a rash on the hands and feet.
Infectious Mononucleosis: This condition, caused by Epstein-Barr virus, can lead to fever and oral ulcers, among other symptoms like sore throat and lymphadenopathy.

Acute Leukemia: Although rare, acute leukemia can present with oral ulcers and fever due to bone marrow suppression leading to immunocompromised states. Missing this diagnosis could be fatal.
Agranulocytosis: A potentially life-threatening reduction in white blood cells can lead to oral ulcers and fever due to the body's inability to fight off infections.
HIV Infection: Primary HIV infection can manifest with oral ulcers and fever, among other symptoms. Early diagnosis is crucial for management and prognosis.

Behçet's Disease: A rare condition characterized by recurrent oral ulcers, genital ulcers, and uveitis. Fever can be present during active phases.
Pemphigus Vulgaris: An autoimmune disease causing blistering of the skin and mucous membranes, which can lead to oral ulcers and fever if there's significant mucosal involvement.
Erythema Multiforme: A skin condition that can also affect the mucous membranes, leading to oral ulcers and fever, often in response to an infection or medication.