My first son had this as well and I researched into my husband's family and found out that a citric acid allergy ran in his family for boys until they were about 10 years old. So, that explained why he was always getting a diaper rash- poor bleeding butt and hives at times too. So, we eliminated all citric acid from his diet (well, as much as we could as it is in almost everything) and it helped significantly. So, you may want to look at what she is eating.
To help the diaper rash, I have tried almost everything (some previous posts here had some things I haven't tried) but I absolutely swear by Weleda Diaper Care Cream (make sure it is the Diaper Care, as they make another one too). You can get it at most health food stores (call first)and on line stores too. It works wonders and their little tushes are better by the morning (well, the bleeding part will scab over and take a few days to heal). I would wipe his butt with a cotton pad dipped in water (wipes will burn), then put on some baby powder to dry it off (that's key, to make sure their skin is dry before putting on any ointment, then put on a layer of Weleda Diaper Care Cream, then put on a layer of vaseline. Do this every time and they won't get diaper rash. My son used to be so bad that we couldn't change him in public because people thought we were hurting him and would actually ask me through the stall door if we were ok, etc. It was bad. I have also read that you can put some flour in the oven and brown it and use that and it works wonders too. I haven't had the time to try that one. But, we have used all the ointments, triple paste, maalox, lotrimin cream, etc. The last 2 work occassionally. You may want to take her to your pediatrician and rule out a yeast infection. Good luck!