Welcome to Healing Well! I also had a Nissen, but mine was a 360. I have not heard of hyperventilating when walking after the surgery. Have you discussed it with your GI doc or surgeon? Perhaps there are others who will be able to share a similar experience.
Here are some websites with information about
hyperventilation, its causes, and treatment:www.medicinenet.com/hyperventilation/symptoms.htmwww.emedicinehealth.com/hyperventilation/article_em.htm
There is just one other thought I had. Some people get irritation or damage to the vagus nerve, which can cause hyperventilation symptoms. Here is a website that discusses it:www.wrongdiagnosis.com/v/vasovagal_attack/intro.htm
I hope these resources are helpful to you. Again, welcome to our forum!