My guess is that the yeast infection hasn't totally cleared up. If she were just adjusting her underwear then I'd say it's a habit, but if she's really digging, she's really itching.
How did you treat the yeast infection? It's possible that the yeast has infected the skin around her vagina, but is cleared out from the inside of her vagina. That itches like crazy, but not all doctors catch it. Try another week of anti-fungal cream just applied to the skin around her vagina.
Also, as another poster mentioned, how's her hygiene when she pees? My daughter was CONSTANTLY picking at her underwear and it was driving me crazy. I brought her to the dr to check for a UTI twice, and it was negative both times. Finally I brought her to a urologist, who told me to a) have her stop taking baths, always showers and b) make sure that she pulled her pants all the way down to her ankles, rather than knees, when she peed so that she could spread her legs really wide. She then does a little "shake" and then wipes. A week of that, no more underwear picking.
Having been in your place, I really do understand how crazy-making it is, but do check the physical stuff first. It stinks to have an itch you can't scratch. Good luck.