have you changed anything while doing the laundry? My daughter was real itchy (in front though), and I thought yeast infection, UTI... had her looked at.. nothing. Then I realized I had tried a different detergent and I had been using a lot of oxyclean because her sister was potty training. haha!!! I went back to my normal soap and stopped using the oxyclean, and she stopped itching shortly thereafter.
now my other daughter is scratching herself, and took her to the Dr last week... everything is fine. I just went and bought All free and clear, and i will see if that helps. the Dr also recommended using some diaper rash cream (even though she is potty trained) because it will protect her bottom if there is something irritating her, or another Dr has given me a recipe of mixing a little anti-itch cream with vaseline.
also try toddler wipes (or baby wipes) instead of toilet paper... maybe that will help too.