H.W.
I know I've said it before, but Hanna Andersonn makes very comfortable kids underwear that actually fits correctly. You might try going online with her and letting her pick out a few pair.
Also, if it happens that they are bothering/irritating her, did you perhaps change detergents or begin using other laundry products? I'm super-sensitive to soaps and fabric softeners, etc., so that might be a culprit. I had to rewash all my clothes in just hot water with no soap to get them 'clean'.
This could also just be a phase. I know plenty of very nice adults who cop to going 'Freestyle' under their pants, and there's nothing wrong with them, per se. It's just a preference.
I am learning, though, on this site, that some parents feel Very Strongly about their child wearing underpants. My guess is that if you work with her, this huge opinion of hers might just go away on it's own, esp. if her peers give her their opinions!