What Causes Excessive Mucus In The Throat After A Gastric Bypass Surgery?

I am 6 weeks out of my gastric bypass surgery. Recently had an upper GI procedure done because I have been nauseous and vomiting for 4 1/2 weeks, cannot eat any food and even vomit most fluids. The GI results showed almost a total restriction where my esophagus meets the bypass. Two days ago my surgeon did an upper endoscopy to inflate a balloon to stretch that area. Those results were, he says, that I had a large amount of mucus in my throat. He said he believes the mucus is the reason for my nausea and vomiting and it s due to allergies. I ve never had allergies in my life. He said it could be from something else, but he believes allergies. What do you think the mucus is from? What else causes that much mucus since surgery?