Hello, have you found out what it is? Same thing happened to me over a year ago and I've seen numerous Drs/specialist, been to the ER, had Ultra sounds, x-rays, CT scan, MRI, numerous blood test, EKG, Echo.......and no one can figure it out. One specialist felt it was a damaged lymph duct from a thyroid lobectomy 2 years ago but MRI came back normal. All Drs see the swelling but no one knows why and or what it is.