As it turns out, even seemingly "random" strays are nothing out of the ordinary. "We are mammals and our bodies are covered with hair," said dermatologist Dr. Purvisha Patel, MD. "There are hormone receptors attached to each oil gland and hair follicle of our bodies."
When you have a period, your hormones flux and change. (You probably know that affects your skin because you might be used to your skin getting oily or dry around that time of the month.) According to Dr. Patel, those same changes also impact your hair follicles. When you ovulate, your body will produce slightly more testosterone, which might override the estrogen receptors that regulate how much hair you grow. "That means you could see more hair in areas such as the upper lip, chin, neck, or lower belly button," Dr. Patel said.
According to dermatologist Dr. Sara Greer, MD, other hormonal factors such as pregnancy, menopause, and stopping or starting birth control may also contribute to stray hairs. "Even extreme stress could cause it," she added.