T.H.
Hi!
I have a one-year-old who did the same thing when she was your son's age. In fact, she STILL does it. At first it scared me so much- I thought she might have asthma or something worse. I talked to her pediatrician about it and he said she was perfectly healthy.
As she got older, we FINALLY figured out what it was- she stopped breathing, arched her back, and gasped when she was going to the bathroom while she had diaper rash! Babies have that reaction when something is hurting them. Her diaper rash wasn't even bad and she'd do that. Does your son have diaper rash? Even a little bit? Or maybe he has a Urinary Tract Infection? My daughter had that reaction when she'd get a diaper change, while I was nursing her, and what seemed to be at random times. Later on I figured out it was when she was wetting herself. She has really sensitive skin, more so than most little ones.
Keep in mind that babies usually urinate/have a bowel movement WHILE they are nursing. Mine did, anyways. It's supposed to be a natural reaction. My guess is that while your son is nursing, he may be wetting himself (or doing something else that hurts).
Hope this helps!