J.D.
J.- you poor things! My thoughts would be a food allergy, mostly because you said the rash is recent and you may have introduced a food that she'll happily eat (ie carrots) but her body is having a reaction to..A good friend of mine has a dd who pigged out on cherry tomatoes last summer and they were so happy-something healthy, straight from their garden. And assumed that she wouldn't enjoy them if she had an allergy-low and behold horrible diaper rash as a reaction to eating too many. Definitely talk to her doc, as it could be something else. Poor lil' thing. Most of my friends swear by vaseline all over the bottom area right after the bath when her pores are most open. My dd never really suffered with these; I've always used A+D ointment on the smallest of redness and it's gone in the am.
best of luck