What would cause a young women to have low estrogen?

Premature ovarian: Failure, or a problem in the hypothalamus or pituitary that leads to decreased estrogen stimulation. In some cases, extreme exercise, particularly with weight loss such as seen in an eating disorder, can also cause this. Ingestion of anabolics can do this as well. Less commonly, a prolactin secreting pituitary tumor can do the same thing. Check w/your physician.