3 hr glucose test diet and feeling awful

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I have been doing the diet for the 3 hour glucose test since Friday and I have been feeling awful. I have been getting blinding headaches at night and my resting heart rate is through the roof, it was 120 this morning. I have an appt with my doctor in the morning to get checked out but I was wondering has anyone else had this experience that had to take the 3 hour test.