Woke up to one of my girls like this... 😢

Noticed over the last few days that she was acting weird. She was always sitting down. I recently added plastic panels over the entire backyard so that it wouldn't get wet anymore and smell.. because my neighbor's threatened to call to help the apartment even though chickens are legal where I am.... But adding that made it much hotter... I noticed their coop was also very hot at night and so last night I chopped open the wall and added mesh to make it a window. This morning I woke up and I found her like this... All other 19 chickens are fine.. do you think it could have been something to do with the heat? Or was she already sick for a while.. again, I noticed her acting strange and constantly sitting down the past few days.. checked if she was egg bound but saw nothing... Her poop was yellowish but didn't see him crazy regular.. what could've happened?

Thanks in advance and please don't bombard me with "your terrible etc.." she was 5 years old and survived many summers before