Could have been any number of issues. How hot is it? Heat will kill chickens,if in future any chicken shows signs of heat stress(hard panting,wings spread,lethargic,not eating/drinking) immediately submerge them into a pail of COOL water,do not ever use COLD water(cold will shock her)this will drop body temp and will save lives. Could she have had impacted crop,egg bound,vent issues,as you can see it could have been anything. Always look at your chickens poop daily,doing this will alert you to the fact that something is wrong,the condition of their poop will indicate if there is any problem.
The chicks are probably very afraid to show themselves because it's very likely they will get the snot beat out of them. Very dangerous to place chicks in with existing, mature flock. I don't integrate chicks until they are the same size as the older birds.
Way too hot for chickens. Chickens do not have any sweat glands,they rely on their respiratory system to cool themselves down,that is why they pant,spread wings,etc. Their body temp ranges from 103-107 degrees Fahrenheit depending on age of chicken. If their body temp reaches anywhere around 113 degrees and up,they are in immediate danger. You could submerge your girls in cool water as described in my post above. Your girl probably died from heat stress. Hose down your coops to cool everyone off. Look on forums for ways to help your girls stay cool,some are fans,frozen water/milk jugs placed in nest boxes,water bowls,around coops(chickens will lay against them)frozen watermelon/other fruit,hose coops down,small child's pool with enough water to get feet wet. Have chicks been recently introduced,if yes,make sure no other hens are picking/attacking them,this can and does happen.
