There are some reasons that estrogen may be low but the best part of having this as your "problem" is that it is a relatively easy fix with the right herbs. There are many, many herbs with estrogenic properties (much harder to bring up progesterone levels without using progesterone itself in a cream form). Susan Weed has a very good book on using herbs but there are others mentioned on this website's bookshelf as well.
Evening Primrose, red clover, false unicorn root are just a few of the more estrogenic but there are plenty more to choose from and everyone is different so you may just want to try a couple at a time and see what you feel like.
You may also want to check progesterone levels at cd23 just to see if you have unusually large levels and this is what is causing the imbalance (since estrogen and progesterone have a delicate balance). Too much exercise, too low of a body fat percentage (estrogen is stored in our fat cells), excessive stress levels (can lower estrogen and progesterone levels by converting to cortisol), or PCOS, can all be factors to low estrogen.
Most of all, do not give up. It can take some time to find your healing and knowing what needs to be worked on is the first step -- it is valuable to know that it's low estrogen because now you can come up with a plan of action!
All the best to you, Hadley