I am currently in a fitness boot camp (which is great by the way). But today after doing a crunch up type of ab work, feet elevated, arms straight in air and lifting shoulder blades up off ground - rep after rep 50x. When I stopped and got up - I got an immediately sharp headache pain - lasted about 5 minutes. I am 47, female, 180lbs, good health, just out of shape. Am I doing the exercise wrong? Why did that happen?