Pain in the nail of the big toe is commonly caused by the following two reasons: 1. It may be caused by trauma. Usually the big toe accidentally kicks a hard object, or is accidentally hit by a heavy object that injures the big toe, thus causing a local soft tissue contusion, which can also lead to pain in the area of the nail. If the contusion is strong, the nail may separate from the underlying nail bed, resulting in bruising under the nail. 2. It may be a nail infection, which is usually caused by bacteria invading the local wound after the soft tissue or epidermis near the toenail has been injured, resulting in local inflammation. Usually, in addition to painful toenail, there is also redness and swelling in the surrounding area, or even inflammatory discharge or pus in severe cases.