Sorry to hear about your follicle foibles. There are indeed many possible causes of this condition. Your doctor's recommendation of a multi-vitamin is indeed good advice, and yet, though I'm not a doctor, I am relatively skeptical this is going to turn out to be a ready cure, given the severity of loss you describe. Dietary factors that could influence this condition:
An excess of vitamin A or a lack of Iron are two possibilities. The most plausible dietary cause of such an extreme loss, that I can think of, would likely be a gross excess of the trace mineral Selenium, but dietary overdoses of Selenium are very rare, and in any case, your fingernails would to fall out as well.
Inadequate protein in the diet is another possibility. A lack of certain dietary proteins also often go hand in hand with inadequate dietary fat. Vitamins A, D, E and K are all fat or oil soluble. Vitamin E is a natural antifungal, and inadequate Vitamin E in the diet or the ability to absorb such can lead to unhealthy skin and help promote the growth of fungal skin infections. This is another possible cause. You might try washing your hair with tea tree oil shampoo for a while and see if this helps. Tea tree oil is a very potent and safe natural anti-fungal. Nature's Gate is an excellent and highly effective brand available in most whole food markets and any decent health food store. You could follow this up with a good vitamin enhanced conditioner. Try not to overwash your hair though, as the oils you strip out are the body's natural defense, and though conditioner can be somewhat good, it is certainly no replacement.
Lastly, in the nutritional circle, there is always a chance that Vitamin K could be a round-about factor. Vitamin K is essential to blood clotting, and any deficit of the blood's natural functions could possibly lead to poor nutrition of the scalp, thus leading to poor conditions for hair growth. Vitamin K is largely produced by bacteria in the intestines. Any condition that causes a disruption of the natural balance of intestinal flora could theoretically impair the synthesis of such.
As far as exercise, I'm not really sure how this would play any major factor, other than the maintenance of overall health, lowering of stress hormones such as cortisol, and getting adequate exposure to sunlight. Sunlight being necssary for the synthesis of Vitamin D, which in turn is necessary for the absorption of Calcium.
One final dietary factor worth noting under specific conditions is Iodine. Most people get adequate amounts of dietary Iodine in their diet, though if you do not salt your food, or use non-iodized salt, there is always a possibility you are deficient. Iodine is a critical component in the manufacture of thyroid hormones, the thyroid being a major regulator in functions of the endocrine system. The inability to produce sufficient thyroid hormones can lead to a condition known as hypothyroidism. This can occur for various reasons, not necessarily dietary, and can be a relatively common condition, often manifesting in women your age. It is not generally a serious condition, and can be easily treated with pharmaceutical supplementation, given that Iodine is not the factor. This condition definitely has the potential to cause the extreme overall hair loss you describe, and is definitely worth inquiring about or checking into if nothing else seems to help. Particularly if you notice any unusual weight gain, fatigue, or unusually dry skin.
Other possible, more transient factors include the use of birth control pills, and any relatively recent, more severe illness such as a prolonged case of the flu. Hairs are normally at different stages of growth at any given time. In the case of serious illness, you body would tend to down-regulate more non-essential functions. Within months following the illness, hair may begin to appear to be falling out more than usual, due to more closely synchronized growth cycles following the eventual resumption of such formerly depressed processes.
The two final common factors that I could think of, would be: Overtreatment of the hair, causing it to break off near, but visibly without, the root. And lastly, a condition know as traction alopecia, which could theoretically occur if you frequently wear your hair in a style such as a ponytail. Pulling the hair back too tightly for prolonged periods, can in some cases cause damage, resulting in the hair subsequently falling out at the root rather than breaking off as would normally be expected.
Other than cancer treatment or relatively recent childbirth, this is really about all I can come up with at this time. I hope this is of some help in narrowing down some possible causes, though I'd gladly try to answer any other questions you might have. Good luck. Hope things improve soon. Take care.