I'm not quite sure how the Garmin works, but I know some HR monitors will figure a heart rate based on the time lapse between two beats. So, your heart beats only twice, and it gets a rate. If you have an arrhythmia, I suppose the Garmin could pick this up and record it (computers don't account for irregular heart rates). I would suggest stopping and manually taking your heart rate.

Maybe you should get set up for an EKG to get a baseline reading.

Also, I honestly feel that your heart rate should not go above 190 or so. There are limits.