"Hormone levels fluctuate when there are changes in weight-particularly in women. For most, the menstrual cycle is not affected, but some women experience increased activity and break-through bleeding may occur. Others may find that their periods stop for a while.
Once target weight is reached and weight loss ceases, the metabolic rate will adjust to a new level appropriate to the new weight, and the hormones will settle down and find their own pattern again."
Hope that helps.