What is the cause that i feel so tired ?

Many possibilities: Causes of fatuge are many. Most obviously, no getting enough sleep, but if sleep is ok and not feeling rested, sleep apnea, anemia, stress, depression, chronic anxiety, cardiac disease, anemia, medication effects, thyroid are just some possibilities. Since it is a common but problematic symptom, seeing your doctor and starting with a check up and bloodwork is a necessary first step to diagnosis.