Tinea versicolor: This is a common fungal infection that can cause small, circular, brown lesions on the skin, especially in young adults. The sudden onset and appearance as freckles make it a likely diagnosis.

Melasma: Although more common on the face, melasma can appear on the hands, especially in individuals with darker skin tones. However, the sudden onset in this case makes it less likely.
Lentigines: These are benign lesions that can appear as freckles and are common in young adults. They can be triggered by sun exposure, which may explain the sudden onset.
Contact dermatitis: This is a skin reaction that can cause brown lesions, especially if the individual has been exposed to a new substance or allergen.

Malignant melanoma: Although rare in young adults, it is essential to consider this diagnosis due to its potential severity. A sudden onset of new, unusual lesions warrants further investigation.
Addison's disease: This rare endocrine disorder can cause hyperpigmentation, including brown lesions on the hands. Although unlikely, it is crucial to rule out this diagnosis due to its potential severity.

Erythema dyschromicum perstans: This is a rare skin condition characterized by brown, circular lesions, often on the hands and face. It is more common in individuals with darker skin tones.
Phytophotodermatitis: This is a rare skin reaction caused by exposure to certain plants and sunlight, resulting in brown lesions. It is essential to consider this diagnosis if the individual has been exposed to new plants or substances.