I bought a rather expensive snake a while back and she had similar marks on her chin. Breeder said it was from rubbing in the bag (yeah right). I think its from pushing around the tub if there's even the slightest gap or the snake is a very active snake. Mine got better every shed but after three sheds you can still see marks. Not open sores just marks. Ill post a pick and show you when I have time. (At work right now) Let me know what the vet says cause I'm very curious also. Marks looked exactly the same just not quite as bad. Now they're almost like little bruises. Hopefully she'll clear up with a couple more sheds.
Sean