When one of your chickens dies and you don’t know why, do a necropsy. It will not only explain the cause of her death, but also detect things like viruses, bacteria, and parasites. You will learn from it, and possibly help prevent ailments in the rest of your flock.
Place her body in a plastic bag and tightly seal it to prevent any leaks. Place the bag in a second bag and add frozen cold packs in it. Then bring her to your vet or send her to a lab. If you aren’t ready to do this immediately, you can store her body in a fridge (not a freezer as it will damage the tissues) for a couple of days.
Here in California, the easiest and cheapest ($25 for up to 2 chickens) way is to send her to the CAFHS (California Animal Health & Food Safety). Download and fill out the submission form (note that the first page contains the detailed instructions on how to package the bird).
Here is an example of the form to show what fields to fill out to request a necropsy. Put it with the body in a box and ship it via FedEx Next Day Delivery. Call CAHFS beforehand to get their FedEx account number as they have a very good discount.
It usually takes less than a week to get the necropsy report back.