My buff orpington of 2 years died suddenly this afternoon. She was fine yesterday and was out grazing. This morning I did not notice anything wrong, but I opened up the door for them to range while they were still roosted. When I returned home this afternoon she was in the coop while all the other hens were out in the yard. She was breathing hard with a little noise on her exhales and she would not stand. I carried her into the old doghouse we use for brooding to isolate her from the rest and she plopped down right away. Within the 20 minutes I left her in there to go clean the coop and sanitize where she was laying, she died. Any ideas what may have killed her? Thank you!