When was the last time you got a hair cut? Sometimes when my hair gets longer, I feel like I'm losing a lot but it's really just longer and not more.
Also, have you worn your hair up a lot recently? Wearing it up can prevent hair from coming out through out the day and when you do take it down, it all comes out at once.
If you think it's the vitamin, then try not taking it for a while or switch to another vitamin and see if it continues. I take Centrum and have never noticed a problem with that.