GSP Hiding?

What would cause GSP to go into hiding for 4 days?

I have a colony of GSP that I've had for about 6 months, and it has always opened up fully everyday when the lights are on.  Five days ago the GSP was shrunk up inside so that all you could see was the purple "body".  It remained that way, and I kept checking as much as I could, for 4 days straight.  Today it appears to be back to normal.  I don't remember making any changes or additions to the tank.  And all of the other corals looked normal and the same as they always had. 

Yes, I've seen mine react that way too.  But it always comes back out after a short while.  This was closed up for 4 solid days.

Mine does it for days on end, then comes out and all seems well.  and it does it periodically.  At first i worried about it, but I have about 4 different large patches, 2 patches will be open while 2 others are closed, so I know its not the tank conditions.