Can you imagine two physical or chemical changes that might occur in excercising leg muscles that might trigger vessel dilation and cause increase in blood flow through the capillaries in the muscles

During exercise,  the production of heat by the muscles would cause vasodilation of the blood capillaries so as to dissipate excess heat. This also increases the amount of blood reaching the muscle to meet the increased demands for nutrients and oxygen. The activity of muscle on these vessels also increases local blood pressure without affecting overall blood pressure in the body. This ensures there is requisite perfusion in the muscles

She should ice it and put ointment on it. thats what my momma does.