After I had cold now when go to sleep my throat becomes very allergic and so I cough every 5 min. ?

Could be sinus prob. A cough that occurs following an upper respiratory tract infection can often be due to an otherwise unrecognized bacterial sinus infection. A cough occurring only when laying down could be due to atypical or silent reflux. You also might have some "reactive asthma" which can occur following a cold. See your doctor to have your lungs examined.
Cough. After a cold you may have developed a sinus infection with a post nasal drip that irritates your throat or perhaps the cold was really an allergy and you are having allergic problems. Best see your physician for a physical exam to determine the cause.