alarm keeps going off for no reason?

i have a vauxhall astra 2001 1.6 club over the past 4 days the alarm has gone off for no reason. i lock the car with the key fob and it will go off within 15mins of lockin it, i have checked all doors and windows are shut properly. i cannot even lock the car with the key as the alarm still goes off. any suggestions as to what is happening?