This could be due to a urinary tract infection or it might be due to a yeast or bacterial infection in the vulval folds area. Dampness from urine or washing there can allow surface growth of bacteria or yeast.
If your dog is not spayed then it may be she is coming in heat. That leads to vulval enlargement and the licking is to clean up discharge from the heat cycle which for the first few days is reddish in color.
For the first two situations you'd want to see your vet.
If its a heat cycle starting you've got about 3 weeks where she will need to be kept away from all males and not left alone tied out or in a yard.
Hope this helps you!
I would suggest washing it with cool water and a mild dog shampoo with no insecticide or a Chlorhexiderm or Malaseb shampoo if you have one on hand.
You can then apply some neosporin or triple antibiotic ointment to the sore looking area or even petroleum jelly to protect the skin from any acidic urine.
I'd plan on a vet visit for her to find out what is causing the irriation as she may need oral antibiotics if she has a bladder or kidney infection triggering this.
And in my experience active yogurt doesn't do one darn thing for yeast infections despite all the places that claim it does so I wouldn't bother with it. Besides dogs can be intolerant of dairy once weaned so its not a great idea to give it to them.
Hopefully washing and ointment will help - the ointment won't hurt her if she licks it.
Thanks for the above accept!