Hi you should see the ENT again or go to the hospital to have it drained - it sounds as though you had your drains removed too soon and the fluid has no where to go. I had a similar problem with abdominal surgery and the dr had to drain me a few times to empty the fluid out.

also your neck will swell after a neck dissection - as your drainage system ( lymph fluid ) has been compromised, not now but at some point you may want to consider lymphatic massage. If they are suggesting radiation and chemo I would wait until after you've finished treatment en go, for the massage otherwise just wait until you've healed fully and then go. Eventually you will learn to do it yourself and can continue on your own until new pathways have been built. You will also need physio for your arm and shoulder, best of luck.