I am so sorry for your loss. If you would like to find out what was wrong, your state vet could do a necropsy if you keep the body cold, but not frozen.
Can you look at the picture at 10 o’clock where she has a red mark? Could that be a sting or a peck mark? She doesn’t need to be separated, since the others would have been exposed at the same time she was. Have you added any new birds to the flock recently, or have there been wild birds eating...
Swelling in the face can be from a respiratory or sinus infection. Have you seen bubbles in one eye, nasal drainage, crackles while breathing, sneezing, or mucus from the nostrils or beak? Are there any peck marks near the swelling?