decreased heart rate during exercise

I am 42, female, non-smoker, non-drinker, obese with lots of heart disease and hbp in my gene pool.  I started at 305 and am down to 280 by walking and watching what I eat.   This Christmas we bought a treadmill and I have noticed that I start each workout at a pulse rate of 69-71.  During the workout, it goes DOWN to at least 59-63 and sometimes even lower.  Should I be concerned?  I get it up to 3.5 miles per hour and add an incline and I cannot go faster than my starting rate!