Why/how did my samsung galaxy core prime go from 23% free storage to 11% free storage?

[AC Question]

For the past few weeks, I've noticed my phone had a lot free storage at 23%. However this past week it's been weird going from 23% to 22%, 21% to 20%, 19% to 18%, 17% to 16%, 15% to 14%, and now it was at 13% then 12% and now 11% free storage with a warning exclamation point. Why is this happening when I don't have many apps installed on my phone & is there anything I can do to fix this problem ?

 

[belodion]

What is showing as occupying storage in Settings, Storage, and have any of the categories been increasing as the free storage has been decreasing?