I'm sorry your little one is not feeling well. Your baby has been on antibiotics? This can really mess with digestion in itself. I too would make sure the rash on your baby's bottom is not yeast. Lotrimin cream should usually take care of it if it is yeast(I often had to use this on my daughter when she was a baby and had antibiotics). Also, something that is really great for CF'ers to take is Culturelle. It is a probiotic(which replaces the good bacteria that antibiotics kill). I started my daughter on it when she was 3 years old or so with the doctor's approval. Not sure about little babies, I would check with your CF doctor before starting it. My last suggestion is to check your baby's stool for C.dificile if the pooping doesn't let up. I would also let your baby's bottom "air out" as much as possible. My daughter would have a sore bottom and to let a wet diaper on just makes it worse. Laying the baby with the bottom exposed and on cloth diapers opened(in case he goes) to let the air get to his little bottom.