I wondered about your question. I have no idea, but I was looking around the net and found someone that had the same problem and another person responded with this answer, but I don't know if it's accurate or not:
Is your dog pregenant or nursing? My dog does it too but she is nursing pups and the vet said she does it because her nipple may be clogged and it probably itches and/or hurts so she sucks it herself to unplug it so she can feel relief and also the pups can get milk from it.
Why does a dog suck their own nipples?