My first thought is you could be UTD, and my 2nd is that your thyroid might be playing up. I also notice your ticker shows you are losing weight. How are you doing this (ie atkins, slimfast, or old fashioned diet and exercise). If it is a restrictive diet then a sudden loss in energy could be attricuted to your body lacking a particular thing, like protein or carbs.

It could also be a virus or something, sometimes the only symptom you get is tiredness.

Regardless of the cause if you still feel like this in a week then a trip to the GP might be in order

T
xx