It is absolutely normal for children to touch their privates. They do it because they're curious and it feels good; it does not necessarily mean that there's a rash or other health problem going on. She is at an age where you should be able to talk to her about what we do in public and what we do in private. But you have to be careful to make it very clear that there's nothing wrong or "dirty" about touching herself, just that it's something we do when we have private time. You should also explain to her that she needs to wash her hands afterward, and that if she touches her butt she shouldn't touch her vulva again until she's washed her hands, because there can be germs there that could make it sore. If she's learning how to wipe herself you can also explain that that's why she has to wipe front to back.
She may have a sensory issue with the panties. If you haven't noticed any other symptoms I wouldn't think she has full-blown SPD, but just this one issue. http://www.spdnetwork.org/ if you've never heard of it.
You could try boys underwear, or boxers, or bicycle shorts and see if that's more comfortable to her. Also, make sure you're not washing them with fabric softner or scented detergents, and give them an extra cold rinse cycle.
Disposable pull-ups may make it harder to pottylearn because they don't have the sensation of being wet and uncomfortable. You might want to try cloth trainers and see if she can tolerate them.
Can you ask her care provider to take her to the potty every hour?