I worked out on the treadmill yesterday and with the same settings I usually use my heartrate right at the end was 125 (the machine takes the heartrate). Usually I am at 150. What could cause such a drastic difference? Normally I wear a hat to keep the sweat out of my eyes, but this time I forgot it, so that was really the only difference I could tell.
As you get fitter, you will have to work harder to get your HR higher. So, the effort to raise your HR to 150 will increase as you get fitter. However, 25 is quite a drastic number lower in one day. Did you check your HR before you start? Perhaps your HR was already elevated for one reason or another, possibly that cute chick on the next treadmill. Check it again today or tomorrow.
You could be tired. If you have been working out often and not resting enough, this is exactly the sort of thing that you can expect. Take a few days off and you’ll probably find your heart rate back in the usual zone.
It could simply be a malfunction in the treadmill’s sensor. Try taking your pulse at your neck rather than relying on the machine. Count the beats for six seconds and multiply by 10.