My daughter, who is also 7, has been getting yeast infections since she was 3 or 4. The doctor just thinks she's sensitive down there, and tells us not to use soap at all on her skin, and to use Aveeno Baby lotion if she needs it and a very gentle shampoo. She can't have bubble bath. She is not supposed to stay in the bath long (showers are better but she doesn't like them), and needs to get very dry as soon as she gets out. She begged and begged for the kids' scented shampoos, and I just bought her one the other day--I was skeptical, but it said "extra gentle" on the outside. Sure enough, she is itching down there again...
So, try to keep her as dry as possible when she gets out of the bath. Feed her yogurt. (If you're using vitamins and probiotics, you're probably already doing that). Antibiotics can cause it, but you've said she's not on them, and my daughter gets them even without antibiotics.
My daughter also seems to have soap allergies, and they said the reaction has been bad enough to cause a yeast infection (for one of them), so the allergist visit probably wouldn't hurt.