Talk to Dermatologist on Hair Fall

What could be the reason of excessive hairfall during the age of 18 , last year during the summer I had the same problem, then i trimmed my hair and a doctor told me that I have blood and vitamin deficiency and he suggested me multivitamins and some other medicines. By taking those medicines my hairfall somewhat stopped. But this summer again I'm facing the same issue but this time it's too excessive I don't know what to do