This is either a dragging parking brake or a stuck brake caliper piston. You need to jack up the wheel and turn it to see where it is dragging. You might have to pull the disc off to see if the parking brake parts have come apart.

Make sure you chock all the wheels before lifting one of the rears.

If this starts to really drag you may not notice the drag but you will soon notice the heat. The heat will discolor your alloy wheel by burning the paint off of it.

You might just need new brake pads.

Pooka