You can clean the pasty butt off (if they cannot poop they will die so it must be washed off) by running warm water and putting her backside in the warm running water until it softens. Then remove it with your fingers. Of course wash your hands with soap afterward.
OK do you have other chickens who are outside? I am thinking there is always the possibility of coccidiosis at that age, however, the cocci come from the soil and would have to be transferred into the brooder by equipment/hands etc.
The other thing to check is your heat. Check your temps under...