I get spontaneous bruising of nails on great toe. why couldit be?

Minor Trauma?: May be due to trauma caused by tight fitting shoes. Some athletes will have bruising if their toes are striking the top of their toebox of their shoes.

Probably shoe cause: Usually due to minimal trauma from shoes or boots. Referred to as subungual hematoma. Should be cultured or biopsied to rule out fungus or in rare cases can be cancerous (subungual melanoma).

Bruise: This is seldom spontaneous. See your podiatrist for an exam to determine the cause.