Sarah, thank you for your concern, but this actually happened to me over 4 years ago. I am now well aware of the dangers of losing this much weight!
Even after I regained enough weight and body fat, my period still had not started on its own. My estrogen and other hormone levels were far too messed up. It was my ob/gyn's suggestion for me to start a BCP.
I did not have anorexia (I consulted with a physician and a psychologist, and I did not meet the criteria in the DSM-IV), but I did severely restrict my diet and exercise excessively. Thankfully, I've since sorted out all of the misconceptions that were going on in my head...but it's so sad what women do to ourselves sometimes.