HR is funny that way. Depends on other stuff. But if you have any doubts or even just question about whether your heart is in shape for you to get in shape, then you should talk to a heart doctor or other healthcare professional.

If you really were pushing as hard as you could and your HR didn't go up, then that might be a health issue. How did you set your HR zones?

Being a little dehydrated can cause your HR to beat faster than is normal. And of course caffeine and other drugs factor in too.