Long time no chat.
When did the vet look at her mouth yet?
Trauma or dental disease are the top possibilities. The vet should take a look. Ellie started losing adult teeth. It was severe periontal disease. All better now. This isn't a crazy occurrence. No reason to panic.
When was her last dental? Has she ever had dental xrays? Did it break off or totally fall out? What chews does she get?