I'm so sorry for your loss. We all understand how upsetting it can be.
Was your Lucy's comb normal colored, pale or bluish? Maybe it was her heart?
Sometimes they just pass so fast, and we never really know why.
Just watch your other birds. Make sure there is nothing bad around, moldy food, etc.
But the rest of your chickens may be sad too. So just give them extra loving, extra treats.