This seems reasonable to me, esp as you saw her self-isolating a great deal, and not seeking out the others. I contrast that with Peanut, whom I see shivering out with everyone a lot but then periodically going back to the coop alone to stand on the heating pad, or back to the coop warmer under the coop, where often Hazel goes with her. You know what you were seeing, better than any of us can say. I think in this case being with you, her trusted rooster, inside and warm, would be familiar to her in a positive way, from last year. Both of you did not know for sure that would be her last night. Practice some compassion for yourself now (note to self here too) and minimize your regrets. She had many days of her chicken friend's and human rooster's companionship. Probably, for chickens, each day is not counted toward some ultimate tally, or thought of as a final special day; each day is the Now. She was with her rooster and warm in that Now. She died quickly and without having to endure more days of cold and pain.