I've had this a couple of times now. I get a light headed feeling, I get hot and can feel my heart beat.

Just had it happen again. I know Fitbit is no expert heart rate monitor. But it just showed my heart rate jump to at least 110 (my resting rate is in the upper 60s low 70s lately). I just sat down. But I had simply walked across the office before. Nothing strenuous. And I've been working out pretty regularly, so it's not like my body isnt used to moving.