You need to go to the dr and get some blood tests done for things like iron and haem stores, thyroid, hormones etc, as there may be a really simple explanation behind it (anaemic, sluggish thyroid, viral exposure)...BUT there are also some more serious conditions which show up as tiredness so a good GP will screen to rule those out.

Next step is to take yourself along to a good naturopath or TCM practitioner, they have some fabulous herby type things to help with fatigue and exhaustion, and generally regulate your whole system to get back on track. My naturopath diagnosed me with adrenal exhaustion, I felt soooo much better after about 2 weeks on the herbs & supplements she gave me. Worth a good go.