Why do i get bad allergic coughing specialy at night ?

Postnasal drip?: Night time cough could be postnasal drip or a sign of asthma. See your doctor for treatment.

Pollens Settle: Most pollens settle at night and the air we breath is more concentrated as the pollens fall to the ground. That means more exposure to allergens and more allergic reaction or coughing in the evenings. Depending on what you are allergic to however, some people have fewer issues at night.

Post nasal drip: The cough is mostly from post nasal drip that triggers a cough by irritating your throat but some with bad allergies can have asthma symptoms during the high pollen season. I suggest you consult an allergist to rule that out. You need a nasal spray to control your drip.