Benign fasciculations or myokymia: This condition is characterized by involuntary, spontaneous contractions of muscle fibers, often presenting as twitching under the skin. It is usually benign and can be caused by fatigue, anxiety, or excessive caffeine intake. Given the negative past medical history and the relatively short duration of symptoms, this diagnosis is the most plausible.

Muscle strain or overuse: Prolonged or excessive use of muscles can lead to myokymia. This could be due to recent changes in physical activity or work habits.
Electrolyte imbalance: Imbalances in electrolytes such as potassium, sodium, or calcium can cause muscle twitching. Although the patient has a negative past medical history, acute conditions or recent dietary changes could lead to such imbalances.
Vitamin deficiencies: Deficiencies in vitamins like magnesium or vitamin D can cause muscle symptoms, including myokymia.

Amyotrophic Lateral Sclerosis (ALS): Although rare, ALS is a progressive neurological disease that affects nerve cells in the brain and spinal cord, causing loss of muscle control. Early symptoms can include fasciculations or myokymia. Given its severe prognosis, it's crucial to consider ALS in the differential diagnosis.
Multiple Sclerosis: This chronic disease affects the central nervous system and can cause a wide range of symptoms, including muscle twitching. Early diagnosis is key to managing the disease effectively.
Thyroid disorders: Both hyperthyroidism and hypothyroidism can cause muscle symptoms, including myokymia. These conditions are treatable and can have significant health implications if left undiagnosed.

Isaac's syndrome (Neuromyotonia): A rare autoimmune disorder characterized by muscle stiffness, cramps, and delayed relaxation of the muscles after contraction. It can also cause myokymia.
Cramp-fasciculation syndrome: A condition that presents with muscle cramps, fasciculations, and sometimes myokymia, often related to potassium channelopathies.