Hi my age is 19 year old.i am suffering from allergic bronchitis.today my pain in left side of head is started.is it due to flu?how i can cure it?

Could be sinusitis: If you have allergic bronchitis you may have congestion in your sinuses and may have developed a secondary sinus infection.see a doctor

Hard to say: without an exam, but given you history of "allergic bronchitis", you are also at risk for allergic rhinosinusitis which can trigger headaches/migraines. If it has only been one day there is always a good probability for a viral infection (cold) so treating it conservatively is reasonable with over the counter analgesics and cold/flu medicines like Robitussin or Benadryl (diphenhydramine).