Lower than normal heart rate when exercising....

I have a heart rate monitor that I wear religiously when exercising. I noticed today that my heart felt like it was beating pretty hard and my monitor said it wasn't even in the zone. This obviously made me lose some motivation as I felt like I was really working hard and I was hardly doing anything. Does anyone know why this is? What causes something like that to happen? I did take the weekend off from exercising but I can't see that being the reason. I could really use some feedback on this. I don't feel very motivated...

Maybe the hrm wasn't set right - was the chest strap wet, etc.? If not, it might not have been picking up your heart rate. I assume this hasn't been the case in the past, so more likely a malfunction of the monitor.

Thanks! I never would have figured that at all. I thought they either worked or didn't work with no grey area. I am going to check that and get back to work! I must say, I was very nervous that something was wrong with me and I was never going to burn major calories again! And that is depressing!