Definition of Unwell

Unwell: Not well or in good health, ailing, sick, indisposed. For example, "I am well-nigh choked with the sulfurous heat of the weather -- or I am unwell." (1826, Journal of Sir Walter Scott)

"Unwell" is also a euphemism for experiencing menstruation. It medicalizes women with the implication that the menstrual period is always a time of illness. It is no longer appropriate usage (if it ever was).