Awww, poor little thing. I'm sorry that you both have had to go thru this, it's very sad & puzzling. If you're able to get a necropsy, that will hopefully give you some answers.
Perhaps you might like to do what I do & save a few feathers from her, to remind you of her. I take some from the prettiest areas, head, neck, saddle and that beautiflul fluffy egg bum! I label a small ziplock or medicine bottle with her name & date she passed.
I'm sure that your sweet, little hen knew how much you loved & cared about her. I'm certain that they all do. Maybe spending a little chicken time with the others will help you to feel better. I usually house out & sit with my girls at bed time, since they're all roosting. I talk & tell them chicken stories about how they came to live with us, how they got their names, how much I love them all & always try to help them...though sometimes I just can't save someone who is sick. The seem to understand, I get clucks & trills from all up & down the perches and it always makes my heart feel a little better.