Pale Comb and Lethargic

Had a 15 month old RIR hen pass in a matter of hours. This morning she was fine, eating and pecking away with my other two hens. When I went out side at 9am her comb and wattle were pale and she was just sitting still. By 11 he hadn't moved and was looking worse so I wrapped her up and brought her in from the heat. She seemed short of breath and I couldn't get her to drink. Close to 1:00 she had what looked like to me was a seizure for like 5 secs. flopped around and died.
My other two hens seem just fine.. Any ideas as to what happened, has anyone experienced the same thing?