What Causes Swollen Lymph Nodes In Neck And Under The Chin?

I I AM 27 YEARS OLD ABOUT 8MONTHS AGO I GOT REALLY SICK WITH THE FLU THEN PHENMONIA, SINCE THEN I HAD TROUBLE BREATHING ALSO. IT HAS GONE COMPLETELY AWAY BUT NOW I HAVE THESE SWOLLEN LYMPH NODES UNDER MY CHIN AND A LITTLE ONE IN MY NECK. MY DOCTOR HAS RAN MANY TEST AND WE HAVENT FOUND A ANSWER YET YESTERDAY I WENT FOR A ULTRASOUND OF MY CHIN WAITING TO HEAR BACK FROM MY DOCTOR ,WHAT COULD THIS MEAN.