What Causes Gradual Decrease In Haematocrit Values?

My hematicrit reading has been ticking down in the last 4 years with my reading now being borderline anemic. My complete blood tests show everything normal including iron. As a precaution I had a colonoscopy which came back normal. What are some possible explanations for hematocrit readings to decrease over the years? Note, I am a 46 year old male.