Fever and dehydration can cause this, but it would be obvious that these were present. Anxiety could cause this, but your heart rate should come down if you can relax for a few minutes. Recheck your pulse. If it remains this high and does not begin to slow down after several minutes, you may have an abnormality of the electrical system of your heart. You probably need an electrocardiogram (EKG). Call your doctor or have someone drive you to an urgent care center or emergency department now.
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