Low ferritin root cause

I recently had my ferritin level checked and it is low at 4. It has been low for several years now and I have been suffering with extreme fatigue for years as well. I have started supplements but how do we get to the root cause of low ferritin? I know it could be one of a multitude of things including no eating enough iron foods, git issues or other disease. It’s overwhelming trying to figure it out when doctors aren’t being helpful.