Could just be the regular shedding cycle. Our girl, Sable, sometimes gets a spot as you are describing when she sheds. It's like she has shed all of the undercoat in that one spot. It lasts a very short time and when shedding is complete and new coat has come in it's back to normal. Hope that is that is going on with you pup.
Mom to: Sable, fawn, floppy, docked Nov 2, 2002
Tess, flashy fawn, floppy, docked (waiting at the bridge)
Therapy (black lab) Dec 2, 1991-Aug 14,2005 (waiting at the bridge)