1. strenuous exercise: if you exercise less at ordinary times, sudden strenuous exercise may lead to muscle or blood vessel injury and bleeding, thus causing skin abrasions.

2. Trauma: Unintentional bumping, squeezing and other injuries during exercise can lead to rupture of subcutaneous blood vessels and blood accumulation under the skin, which is characterized by bruising and tenderness.

3. Purpura: This is a hemorrhagic disease. Due to the decrease of platelet count, abnormal platelet function, lack of coagulation factors, abnormal blood vessels and other factors, blood cells enter the skin or subcutaneous tissue from blood vessels, which can cause skin congestion, accompanied by nosebleeds, gingival bleeding and other symptoms.

The above information is for reference only, and the specific reasons need to be judged according to the actual situation. You can consult a doctor if necessary. Warm up before fitness, improve the adaptability of the body, thus reducing sports injuries. After fitness, if you feel unwell, you should seek medical advice in time.