It could be from stress. ?
Try taking Evening Primrose Oil (OTC vitamins).
I'm sorry I really don't know too much about this subject but I have heard that the more testosterone (a particular type of testosterone hormone) you have, the more prone to baldness (in men & women) you would be. But since your testosterone test came back low (I'm not sure how "low" is low), I'm not sure what it could be (maybe that's why the Drs are stumped, too). But I did find an article online that may interest you:
http://www.americanhairloss.org/women_hair_loss/causes_of_hair_loss.asp
I don't know if this article will help but it may shed some light on the possibility of alopecia. Good luck & I hope you find your answer.