Male or female? 5 months is about the age males would start fighting too - and if it's hot, it's entirely possible to have a heart attack, or be trampled by the other chickens.
If was a hen, then I'm not sure, other than it's possible they COULD get heat stroke. Some chickens won't drink water, if it's warm - so even on a day that doesn't feel "too hot" they could still get over heated if they won't drink water outside that isn't cool.
We've been freezing gallon jugs and sticking them in their wading pool in the hottest part of the afternoon to help cool them off, and putting gatorade bottles that are frozen in the shade/coop for them to sit again... (Granted, for us, it's 103-109 with heat indexes reaching 130-140+ even in the shade.)
Other than that, I suggest deworming them to make sure they don't have any parasites, and knowing that sometimes, chickens just "die" too.
I'm sure others will have a lot more ideas than me - but heatstroke really is a thing for chickens.