I just got my first heart rate monitor a couple of days ago, and I've used it twice during my morning jog. But yesterday it said my average heart rate for the half hour run was 140bpm (which seemed high to me) and this morning it said my average was 125bpm. That's a pretty big difference, and I'm just wondering what happened. The HRM is a Timex T5J031 watch with a chest band sensor. Can you get a spuriously high reading by getting the chest band on wrong?