during exercise a quick change in the exercise position, difficulty or suddenly stop may result in low blood pressure causing dizziness.
With some forms of exercise you may be shifting the areas where your blood flows. Simply standing from a lying or seated position (more often during exercise) makes blood rush and gather down in your legs and abdomen. This means less blood is circulating and getting back to your heart, causing the decrease in your blood pressure.
Cells called baroreceptors near your heart and neck detect this low blood pressure and send signals to your brain. Once your brain hears the news it tells your heart to pump faster to pump more blood in order to get your blood pressure back to normal. Until your blood pressure returns to normal, you may feel lightheaded.
