you can also look into homeopathy: Calm's Forte and Bach Rescue Remedy are 2 options safe for breastfeeding.
Valerian is safe, but it's pretty sedating, so I would personally take it after nursing, and not at a time you need to be alert.
Magnolia bark is another plant based option, but I've yet to see it by itself, you generally find it diet-aid pills targeting cortisol, like Relacore. The only ingredient in Relacore that I wonder about is DHEA for nursing safety. Relacore works REALLY WELL for anxiety (Not so well for weight loss) according to all the reports, so it might be worth exploring that option with your health care professional.