Heart rate low?

I always feel like my heart rate is far too low for my rate of perceived exertion. I feel like I’m pushing hard up hills while running and can barely get above 155, I ran a sub 21 5k which is damn fast for myself and I got to 166, I rolled on the ground for 5 min after that effort. Am I just not going as hard as I think I am or what?! Btw I use a Garmin hmr pro chest strap.