Is my Heart Rate Monitor working??

Hello

I just purchased a Polar F5 Heart Rate Monitor. I have configured it
but I have a slight problem:

When I see my heart rate it remains around 65-67bpm, but occasionally
it spikes to around 120, for a second, and then goes back down.

What does this mean, am I a super hero? Or, alternatively, is it
telling me something else?

Thanks

Tony