Just from the experience of taking my daughter twice to her pedi thinking she may have a yeast infection... both times the pedi reminded me that girls that young do not get yeast infections.
It could be one of two things--soap irritation or a bacterial infection.
Avoid bubble baths and make sure that she/you thoroughly rinse out her privates after baths.
Make sure she is wiping properly after a bowel movement--and removing all of the fecal matter.
Both times that I've been to the pedi with my daughter it's been a minor bacterial infection from "age appropriate wiping skills." Pedi's advice--twice a day wipe down her privates with a warm, soapy washcloth, rinse, apply Neosporin for a week. It's cleared up the "itchies" within a day or two.
If it is soap irritation, then treat with hydrocortisone.
Good luck.
Edited to add: Both times we went to the pedi, they were very professional and talked with my daughter about how no one is to look at or touch her privates and that it was okay that they look because her Mommy & Daddy were there with her. She was not distraught or embarrassed at all. Also, you can go soap-free for washing your daughter's privates too. If there is external redness, I would start with the Neosporin to help ease the irritation.
Kristen--those are typically external yeast infections... not in the sense that most women consider a potty-trained child to have a vaginal yeast infection--that's the explanation from our pedi.