My Age Is 24 Years. In August 2014 I Donated

my age is 24 years. in August 2014 I donated a bottle of blood. at that time my HB level was 16.1 before donation. After that I never checked my HB level. Then I checked my HB level in November 2015. it resulted a level of 13.5. A drop of 2.6 was seen in one year. I want to ask what may be the possible reason and is there any matter of worry about or not.