gray pinwheel appears

I have a gray pinwheel in the middle of my screen, just twirling away. I looked up and saw the computer was prompting me to choose a "guest user", and the pinwheel was going, so I cancelled the guest user, but the pinwheel stayed. It's not the rainbow pinwheel. It's that gray one made of lines, the one you see when you're shutting down sometimes? You know, like when you shut down your iPhone and it appears just before the screen goes black. I've never had that happen on my computer before, and this was out of the blue. All my applications were working fine, nothing was wrong. I checked for viruses, scanned all the drives, and nothing. Evnetually I just shut down and it went away.

I just want to know if this is a really bad sign. You know, like some weirdness to come.