The temperature of the outside air isn't relevant. The sleeping arrangement, even previous to last night, is relevant. At 3 weeks and beyond, chicks will sleep either on mini-roosts, think a cookie cooling rack, below the heat lamp, if one is provided, This does two things. It pre-trains them to sleep on a roost and discourages sleeping in a pile, as it seems some breeds prefer to do, at their own risk.
Chicks without a mini-roost system would preferably sleep in a kind of sleeping circle. Again, not in a pile. The whole piling thing is to be discouraged. Providing enough heat to discourage that isn't difficult.
Again, brooding in ambient temperatures of 20F to 30F is done all the time and is irrelevant, IF the temperature under the brooding lamp is kept at 90F for newbies and 80F for middlers and 75F for 5 weekers. Once I brood more than 12 chicks, I usually create two heat circles, with two lamps, to spread them out.
Could she have smothered? Maybe, but at that age, it isn't as likely. Chicks are bigger and able to wrestle themselves free from others that crowd them. In short, you may never know. Chicks die. Pullets die. Old hens die. Sadly, living things die. Sometimes without any apparent warning. It's sad, but this is an aspect of husbandry and that cannot always be avoided and sometimes cannot be explained, only conjecture and speculation, which doesn't help much.
