Sounds like she may have had ringworm (a fungus, not a worm) and that it resolved - usually it spreads to patchy loss of hair; in dogs though, you may not see the "ring" that is more obvious in humans - you just see a patchy loss. Fungal or skin infections (esp. when there is hair loss) can cause skin color changes. There are other things that can cause skin color changes - so it'd probably be a good idea to get her in, just to get this looked at.
