Sounds like one of several possibilities, or a combination:
1) Medication side-effect. Hair loss can be a side-effect of many medications, including immunosuppressants such as Imuran, Azathioprine, Methotrexate, etc., and also Prednisone.
2) Nutritional deficiency. B-Complex, Silica, Protein (amino acids), Vitamin E, and other nutrients are required for healthy hair, skin & nails. If you are deficient in one or more of these nutrients, the body won't have the raw materials needed to grow strong and healthy skin/hair/nails. Additionally, Vitamin B12 deficiency can cause complete loss of hair (including eyebrows). If I were you, I'd get tested for B12 deficiency. Also, Biotin can help offset hairloss caused by a variety of factors, including medication side-effects.
3) Stress. This goes along with point 2 above, since nutritional deficiencies both cause stress on the body, and also can be caused by stress. A daily multivitamin/multimineral supplement may be helpful.
4) Autoimmune hair loss - Alopecia Areata & others. Inflammatory activity that affects hair follicles may lead to hair loss and/or poorly formed (unhealthy) hair.
Have you tried hot oil treatments on your hair? I have very dry/brittle hair and was told that hot oil treatments periodically would be very helpful. I've had a couple and they do help some for a while. Also, if you dye or perm your hair, or use a lot of styling products, that may be partly to blame because these chemicals can damage the hair and/or follicles and may be contributory to the problem.
I hope this helps - take care,