I replaced the expansion tank four months ago on my 525i 2006 BMW on the message

What is your question?

It stated that my coolant was low kept flashing. I was advised no coolant was in the expansion tank at all. My question, I had my car for 6 years and I never seen this message before about the coolant being low until I replaced four months. Can the expansion tank be default? What else can cause this problem?