On similar lines, when I was anaemic earlier this year (my haemoglobin levels were very low), I was extremely tired, felt dizzy and had hardly any energy.
When I went downhill in the summer and had similar symptoms I first got my Hb levels checked out. When they were fine this time, it became clearer that I entered a genuinely depressive episode.
It is tricky sometimes to know when a physical condition camouflages as depression or vice versa.
I thus agree with Rohag and think it's useful to have a proper medical check-up as well just to rule other possible conditions out.