It is usually caused by temp, too hot-too cold, draft or got scared and piled up on one another. Need to check temp, they should be laying comfortable under light like a chick rug just touching each other. No panting or no huddling. Keep cleanning their bottoms but most inportant need to get them on a Sulfa medication from your feed store. Its for cattle swine and poultry, brand names are Sulmet, Sulfamethazine, Sulquin, Sulfanox etc.but if not treated you can get 3 or 4 weeks down the line and start losing birds or at the very least failure to thrive birds that are stunted.