There was a woman in my Reserve unit who was going through a rough divorce, her brother was killed in a car accident, she was going to school full-time, working part-time and trying to take care of her daughter. She was very stoic about her situation and never complained or really talked about it much. Her hair started coming out in clumps like you describe. When I saw her I was a little shocked, and asked her about it. I had heard of it, but I never knew anyone who had had the problem. Her doctor said it was due to stress, and it may/may not grow back. Some of her hair was growing back, when I moved away, so I am not sure if grew back completely. If you are under a lot of stress, you may want to find an outlet for it, exercise or something.
It can also be caused by certain medications. My mother had the problem with her blood pressure medication once. The doctor changed the medication and the problem went away.
If you don't have much luck with the treatment, you may want to see a dermatologist. Good Luck.