I also used a treadmill for exercise during my recovery, but I never noticed my stomach bloating because of it. I'm guessing you still have some residual gas that is moving around with exercise, and it ends up in front. Honestly, I have no idea...wish I could help. I'm sure, though, that it's temporary and will resolve with time. Keep up that walking...it'll definitely pay off in the end!
Nissen Fundoplication 2/09
"Whatever you fight, you strengthen, and what you resist, persists.”
“Worry pretends to be necessary but serves no useful purpose”
“Accept - then act. Whatever the present moment contains, accept it as if you had chosen it. Always work with it, not against it.”