Multiple Sclerosis (MS): The patient's symptoms of intermittent right-sided facial pain and numbness, altered sensation, and a headache different from her usual migraines, along with decreased sensation on the left side of her face, could indicate an episode of MS, particularly given the age and gender of the patient. MS is known for its relapsing-remitting course and can present with a wide range of neurological symptoms.

Trigeminal Neuralgia: This condition is characterized by sudden, severe, shock-like or stabbing pain in parts of the face, which could fit the patient's description of facial pain. However, the absence of pain triggered by chewing or other specific actions and the presence of numbness and altered sensation make it less likely.
Migraine Variants: Although the patient describes the headache as different from her usual migraines, some migraine variants can present with neurological symptoms, including numbness and altered sensation. The presence of nausea also supports this possibility.
Acute Disseminated Encephalomyelitis (ADEM): ADEM is a rare inflammatory condition but could be considered given the patient's symptoms of headache, nausea, and altered sensation. It typically follows a viral infection or vaccination.

Stroke or Transient Ischemic Attack (TIA): Although the patient denies significant left-sided weakness, any facial numbness or altered sensation, especially in a young adult, warrants consideration of a stroke or TIA, as these can be devastating if missed.
Brain Tumor: A tumor in the brain could cause a variety of neurological symptoms, including headaches, nausea, and altered sensation. Although less likely, it's crucial not to miss this diagnosis due to its significant implications.
Lyme Disease: If the patient has been exposed to ticks, Lyme disease could be a consideration, as it can cause neurological symptoms, including facial numbness and headaches.

Sarcoidosis: This autoimmune disease can affect the nervous system and cause a variety of symptoms, including facial numbness and headaches. It's less common but should be considered if other diagnoses are ruled out.
Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS): This is a rare mitochondrial disorder that can cause stroke-like episodes, headaches, and nausea. It's more common in children but can occur in young adults.

Given the differential diagnoses, the following orders and medications could be considered:

MRI of the brain to rule out MS, brain tumors, and stroke/TIA.
Lumbar puncture if MRI suggests MS or an inflammatory condition like ADEM.
Blood tests for inflammatory markers, Lyme disease (if exposure history), and possibly mitochondrial function tests if MELAS is considered.
Steroids (e.g., prednisone) if MS or ADEM is suspected to reduce inflammation.
Pain management as needed for headache and facial pain, possibly including anticonvulsants like carbamazepine if trigeminal neuralgia is considered.
Anti-nausea medications to manage nausea and improve oral intake.