I lost two girls in a week too
so now I am down to 6 girls.
I have no idea what happened to mine, the first one I lost was 19 1/2 weeks old. It was at 5:30 in the evening.She died so quick, in less than a minute. One minute she was standing there, the next.. she dropped, flapped her wings a couple times, and was gone. Then yesterday morning at 8:00 am, the same exact thing happened to my other girl. She was 20 1/2 weeks old.
I am still racking my brain trying to figure out what happened. Not sure if it's genetics, or what. They both (all mine are) very healthy and happy. No mites, worms, injuries, sores, or any physical signs of sickness. They have a huge covered run, a large outdoor run, with a playground, and free range with me in the afternoons. There is tons of shade as well as frozen grapes and ice in their water when it gets too hot. Yes.. they are spoiled! I love my baby girls, so this has been very hard on me. I don't want to lose anymore, but if it happens, I will take her to the state extension office for a necropsy. If it happens again to you, (please no!!) I think it would be a good idea for you to do the same.
Not sure if the heat would have killed your birds.. it's possible, but they are very tough! Mine get manhandled whenever my neices and nephews come over, as well as me! I love to hold them and love them... They have never showed any signs of stress or anything. So sad, we may never know what happened to them.
I am sorry I'm know I'm not much help! I have searched and searched. The only thing I have found remotely close was sudden death syndrome, or flip over disease, but that happens to broiler birds, when they are very young.
I am so sorry for your loss.
Good luck to you and your babies!