Sorry to hear you're having some problems. Have you eaten more challenging food lately? While the Nissen surgery can reduce reflux dramatically, there is likely some reflux remaining. If you were to eat or drink a lot of very challenging foods recently, it could stir things up for a while.
If your endoscopy shows no irritation, it would indicate to me that you're not getting significant reflux.
Sometimes my wrap area can feel very irritated because of what I've eaten. If food is being held back by the wrap, it can be very irritating and mimic reflux. Are you remembering to continue to chew your food thoroughly? That requirement doesn't change over time. Your wrap makes it more difficult to pass food through into your stomach, so you need to always chew it thoroughly after surgery.
Hopefully things will settle down soon.