Severe Hair Loss. Of this problem. ?

I am a 28 year old female. From the past 1.5 years I am suffering from chronic hair loss. In the beginning, I thought that the hair fall may be seasonal or due to the change in location. But it went o for 6 - 8 months and stopped suddenly in December last year. But in April this year, it started all over again and is continuing till date. My haemoglobin is 11.98 and have got checked for thyroid as well. Reports came back normal. Have taken multivitamins but they were not much help. Have used Minoxidil too on advice of dermatologist, but stopped using it after few days as I was not sure about its continuous use. My hair have lost all its volume and have become extremely thin. Kindly suggest possible cause of this problem..?