I don't think I'd be worried about that. Certainly could be lack of light from shading, could be someone aggrivated it, could be I whacked it with a siphon hose (oh, yeah, that's what I did recently, not you!)
I have one or two Zoes that stay that way and a few Palys that do that after a big meal of blood worms. I even have a Bounce Zoe I call Bounce Bounce Baby. lol
In my book it ain't nothin to worry about.