When I try to calibrate my iPod nano after a run it just shuts down and restarts. Whats happening?

After I run (iin basic mode) I try to calibrate. (I've done this a few times before).

Lately when I do/try this the iPod just shut down and restarts.

I've tried restoring the iPod in iTunes to no help.

It's a 6th generation nano coupled to a nike sensor.

Any help would be much apreciated.