I am going to go a little further. This is probably genetic weak nerves. Sound sensitivity can be passed onto her puppies. You really do not want people who have puppies you sold or gave them to call you and tell you they are having this trouble or worse troubles with their puppy.
Do not believe that some fireworks on NewYear's caused this. German Shepherd Dogs should be capable of working within a war zone. A two year old bitch should not be so flighty that she cannot recover from some loud noises. A reaction is ok, but they should recover right away. For her to then be afraid of going outside, that is I think, indicative of weak nerves, which will be handed down to her progeny but it might manifest itself in several undesirable ways. Sound sensitivity and fear of storms being just one of them.
You may want to inform your breeder of this problem as well.
Genetic weak nerves cannot be cured, not completely, but they can be managed and things can improve. A trainer may be able to help. Building her confidence may be helpful. I think managing the environment and avoiding the loud noises might be helpful at least for a period of time, and then go slow. Try to do things matter-of-factly with her, and not coddle her about going out, but if you can tweak things to avoid things she is having problems with for a couple of weeks it might be helpful to get a break.
Jenna, RN CGC & Babs, CD RA CGC HIC
Heidi, RA CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear CGC