Each hair has a growing lifetime and once it is done, it stays for a bit, then falls out. Here's the hair growth cycle:

Anagen Phase
Your hair grows around half an inch a month, and faster in the summer than in winter. The growth phase, or anagen phase, lasts an average of 3-5 years, so a full-length hair averages 18 to 30 inches. The anagen phase is generally longer in Asians, and can last as much as 7 years with hair being able to grow to 1 metre.
Catagen phase
The catagen phase, also known as the transitional phase, allows the follicle to, in a sense, renew itself. During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests," cutting the hair strand off from its nourishing blood supply. Signals sent out by the body (that only selectively affect 1 percent of all hair of one's body at any given time) determine when the anagen phase ends and the catagen phase begins. The first sign of catagen is the cessation of melanin production in the hair bulb and apoptosis of follicular melanocytes.[7] Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward. While hair is not growing during this phase, the length of the terminal fibers increase when the follicle pushes them upward.
Telogen phase
During the telogen or resting phase (also known as shedding phase) the follicle remains dormant for one to four months. Ten to fifteen percent of the hairs on one's head are in this phase of growth at any given time. In this phase the epidermal cells lining the follicle channel continue to grow as normal and may accumulate around the base of the hair, temporarily anchoring it in place and preserving the hair for its natural purpose without taxing the body's resources needed during the growth phase.
At some point, the follicle will begin to grow again, softening the anchor point of the shaft initially. The hair base will break free from the root and the hair will be shed. Within two weeks the new hair shaft will begin to emerge once the telogen phase is complete. The process results in normal hair loss known as shedding.

If your body is undergoing stress (ie, diet, or workplace stress) the regrowing of the follicle will be 'put on hold' as your body deals with more important physical issues related to the stress. So, hair will fall out as normal, but not be replaced as soon as it would normally be without the stresses. Once stress lessens, hair will regrow. The vitamins you are already taking will help you make sure you have adequate nutrition and any fancy extra supplements won't really make any difference. Hair differences while pregnant and post-partum are due to changes in hormone levels.