It could be any number of things. Or it could that you might have a B12 deficiency. You can always check, by having a blood test done to test your B12 levels, and then getting a shot of B12 for a boost of energy.
A little information on the vitamin!
Basically, B12 is a water-soluble vitamin that helps your body to produce red blood cells. It plays an important part in maintaining your brain and your nervous systems functionality. It even affects the synthesis of your DNA.
Interestingly enough, B12 cannot be produced naturally in animals, plants, fungus or humans. Only specific bacteria and archaea ( microbes which are physically similar to bacteria but posses a different gene structure) are able to synthesis this Vitamin. If a natural food (like Salmon) contains B12 , it is because it has bacteria that live in a symbiotic relationship with it, thus producing the vitamin for the plant or animal. This is known as Bacterial Symbiosis.
This vitamin is often produced through industrial means, by a process called bacterial fermentation-synthesis. Making it more widely available for distribution as an additive to food, drinks, pills, and even in injections!