I just came upon this site last night after researching this same swelling under "supraclavicular fossa swelling" which seemed to describe my situation but I found it bilateral. Saw my primary yesterday and he sent me for a Chest X-ray and ultrasound of my neck and numerous labs. Still waiting on the Ultrasound. My doctor felt the majority of swelling was on the left side . Mine started 4 days ago. I was reading studies by Oxford and a few other sites that popped up and it seemed they were finding this in women 55 ish and older on average. Some had gastritis before, heartburn, or had been gardening and some shoulder pain. There were several recurring scenarios. Now I see that some of you have this recurring. I am waiting to see what my doctor finds as so far all is negative . I am thinking my US results might show something. I have also felt what I describe as pressure when I lie down around my neck and sometimes some crackling high up when I breathe when sleeping. I do have seasonal allergies with congestion. I also wonder if this was tied to a virus. I had influenza B in January and it hit me hard and it took over a month to start feeling better and had to be on inhalers. I never get sick to this degree. I saved the articles to share with my primary. One site suggested paroxysmal thoracic duct occlusion syndrome once other conditions were ruled out. It's all a puzzle. I am thankful people are willing to share their experiences and hope nothing serious comes from this swelling. I am thinking I might be on the same path as you, more testing.