I have a headache at the back side of my head for a month or so?

The most likely conditions that cause symptoms like you described are:

-Tension headache: the most common type of headache, caused by muscle tension in the neck, face, jaw, or shoulders. Many people have occasional tension headaches, which often start in the middle of the day. For others, tension headaches occur every day. Everyday stress, lack of sleep, anxiety, depression, bad posture, hunger, and tiredness can all trigger a tension headache.

-Acute sinusitis is an inflammation of the sinuses caused by infection lasting less than 4 weeks. Sinusitis often develops when swelling, inflammation, and mucus from a cold block nasal passages. This makes it easier for bacteria to grow. Organisms can produce gas, and when your sinuses are not draining, pressure can quickly build and cause the infection to spread. Viruses are the most common cause of acute sinusitis, but bacteria, fungi, and allergies may also cause it. Sinus pain and tenderness are common, but severe pain may indicate an abscess or other complication.

-Migraines are a common type of headache that can cause severe pain. It is the most common type of recurring headache in children. Migraines can last for a few hours or a few days. They can cause throbbing, sensitivity to sound or light, nausea, or vomiting. Migraines may be triggered by certain foods, stress, or changes in the level of serotonin in the brain. Some people have an aura before a migraine, which can have symptoms that include temporary vision loss, seeing stars or flashes, or a tingling in an arm or leg. There is no cure for migraines, but medications can reduce pain or stop migraines from occurring.

-Sinusitis is a swelling of the sinuses that blocks the sinuses from draining normally. It is often caused by a bacterial, viral, or fungal infection. Sometimes sinusitis is caused by allergies or a blockage in the nose. Sinusitis can cause a stuffy nose, bad breath, cough, fatigue, fever, and headache. You have chronic sinusitis when your symptoms last longer than 12 weeks.

-Nearsightedness is a common eye problem that causes blurry, distant vision. It occurs when the shape of the eyeball or cornea -- the clear layer that covers your eye -- does not refract incoming light rays correctly. You may be able to clearly make out the words in a book in front of you but not those on a sign in the distance. Eyeglasses, contact lenses, or eye surgery can solve the problem. Sometimes diabetes or a developing cataract causes nearsightedness.

Get an appointment with a family doctor, a pediatrician (if you're under 18) or a general practicioner and go find out what it really is.