Sudden usage spike

my Internet usage increased significantly from November to December and January. I called customer service a few times on the issue they said there was nothing they could do. I asked to get a breakdown of usage by device they said they did not have access to that information because it’s private. I said if it’s private how do I access my own private information. They said I need to download a third-party app to get that information. They said that if I go over I get one month free and there’s nothing they could do for me. It’s very frustrating  because I was told a family of four would not even come close to using the terabytes but I am one person single and I am coming close to using that data with no explanation or help in finding out why!