i dont know what is causing the the rash but i will give you a few suggestions 
Emma went through a phase were she kept getting yeast rashes and they would get red and blister and it was painful for me to even wipe her with a wet washcloth so what i would do is run water in the tub and stick her bum under the water to rinse her off......this is less painful for the baby and you will be rinsing off off the ickys and not just wiping them around (if it is a bacterial rash caused by something in her poop)
another thing i was told was to use corn starch powder (golds makes one that has cornstarch and zinc which works wonders) with every diaper change b/c it will pull the moisture away from her bum...so try powdering her up good after every diaper change...it will help pull all the water in the runny poop away from her bum....
and lastly try getting her to eat bread and stuff the will bind her up....maybe she just isn't getting enough solids?
Good luck i hope she feels better soon
Emma went through a phase were she kept getting yeast rashes and they would get red and blister and it was painful for me to even wipe her with a wet washcloth so what i would do is run water in the tub and stick her bum under the water to rinse her off......this is less painful for the baby and you will be rinsing off off the ickys and not just wiping them around (if it is a bacterial rash caused by something in her poop)
another thing i was told was to use corn starch powder (golds makes one that has cornstarch and zinc which works wonders) with every diaper change b/c it will pull the moisture away from her bum...so try powdering her up good after every diaper change...it will help pull all the water in the runny poop away from her bum....
and lastly try getting her to eat bread and stuff the will bind her up....maybe she just isn't getting enough solids?
Good luck i hope she feels better soon