+1 to all...

1) Check for a broken axle.
2) Examine the bottom bracket shell and all the junctions of the stays for cracks or other issues.
3) If you have relatively large tires on it on HARD efforts you might be flexing the frame enough to rub, that wouldn't be unheard of. How much of a gap is between the tires and chainstays at rest?
4) Do check for loose spokes but if some were so loose as to cause this CERTAINLY the LBS would have caught them in the truing.
5) Pick the rear of the bike up off the ground, grab the wheel and wiggle side to side, there should be no play.

Let us know how it turns out.