The fatigue and lack of concentration could be due to main reasons.

It could be due to:

a) Anemia - which is reduction in the red blood cells in the body.

b) Low blood pressure

c) Depression can also present with these symptoms.

d) Chronic fatigue syndrome.

e) Stress.

Generally, Caffeine stimulates the brain and increases the heart rate and adrenaline, which in turn keeps you awake. But you have the opposite effect with it. Don't worry about it.

I would suggest you to get a complete blood test and physical examination done and consult your doctor with these reports for further treatment.