Mysterious Hair Loss..plz Help

4 yrs ago I used to work out in gym jus to maintain my body for a year with out using any supplements just regular healthy home food.After that I left the gym n got into my studies..

Last 3 yrs I am havin a sudden problem of hair fall which started a week before my semester exams may be because of the study load(which I dnt think mite be the reason).
My hair grow for few weeks n then fall one of a sudden in large nos enen if I m not having any exam.This is phenomenal n takes place one of a sudden.

I generally dnt use any chemicals just a regular shampoo.
There is no heridetary cause to this as my dad n n bro all have thick hair which I used to have at times.The desity of the hair is sparse n they r thinning while they grow too after a period but when they start falling they loose the shine n lusture n bicome brittle n easy to break types n also are easily pluckable..

The real reason is not known to me as I m not having any bald patches on my head as there is a regrowth but there is thinning of hair.
I am really quizzed as this is goin on frm last 3 yrs n I dnt know the reason why?

1>there is no family history of hair fall
2>hair fall occurs even when I m stress free
3>all my recently taken health report(urine,blood) are normal
4>idont have any other health problem.
5>i am not adicted to any bad habits(smoking or drinking)
6>did show this to the dermatologist he says all is rite jus wait n ull get ur hair back.

Tell me solution to this plz as its been 3 yrs n this is goin on...