I went to donate blood 2 weeks ago and they said i couldnt as my levels of haemoglobin and iron were too low.?

I received a letter in the mail from the Red Cross today saying my Haemoglobin level is 106 and Ferritin is 4.87. What does this mean? Is this bad. I have been feeling rather sluggish lately...could this be the reason and what makes my levels low when only 3 months ago they were normal levels.