I took a "break" from my normal workout routine for a while (exercising only 2-3 days a week instead of 6, for about a month). Now that I am trying to get back to my normal routine, I've noticed that anytime I do an exercise in plank position, I get nauseous. This never happened before my "break." Once I start feeling nauseous, the feeling doesn't go away until long after the stop exercising, which makes doing the rest of the workout more difficult. What might be causing this? I don't think i'm doing anything too different than before. Any suggestions on how to stop it?