The short answer is that I don't know whether there's a way to get past this sudden gag reaction, or why it's happening now. I do, however, have some follow-up questions that might help. Did absolutely anything else change at the time that you abruptly went from easily swallowing to immediately gagging? Your stress levels, diet, or environment?
Some of the most painful moments in relationships don't happen during overt conflict. They happen in its wake, or alongside it, or quietly-when both partners are trying to connect and still feel alone. These are the moments couples struggle to name. Nothing "bad" has happened, exactly, on the surface. No one has exploded or walked out. And, yet, something has gone wrong.