Nauseous in plank position

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?
 
when you 'switched' workouts did you continue any ab work? Also if you have put any weight on in that month in can have a massive effect as it is easier when lighter (no offence) :)
 
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