What Causes Frequent Urination?

Hi , i am a 23 year girl. I have been suffering from frequent urination from almost four to five years. Back then I used to drink lot of water to keep me awake and cool during my college entrance preparation, and I used to think that this frequent urination is due to high drinking only. But as it has continued so many years, I went to a doctor four months before. Sonography took place, and he said there is nothing unusal except that he thinks my bladder is smaller in size , he gave me medicine to take during night and report back after few months. I am not seeing any improvements from medicine though i have reduce my water intake as well to minimal requirement. Also since few days i am experiencing a little faint after coming back from urination(even in day time). I am little scared with this symptom. Can you please suggest me what may be the cause of my problems.