Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
This white chalky stool is usually due to a problem with the bile ducts and or digestive system. Even though Sadie is acting normal, something is going on within her digestive system and/or liver so it is important that you take Sadie to your vet for a complete exam with blood work. Considering Sadie's age, there may be something underlying that could be treated to give her a longer life.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta