HI- this cancer can be nasty - I had the same type of surgery etc (same location too) and same treatment.
My difference was that I had a small 3mm node highlight on the opposite side on my pre radiation CT - after asking about it and pushing them they decided to radiate my entire neck area (all nodes) to be on the safe side even though they were not concerned about my glowing node. (I was - mind you it was my neck not theirs)

drs are often reactive not proactive.

Anyway - with regards to the tightness etc... that should change on the non radiated side eventually - stretching should help (since non radiated muscles have been cut there will be scar tissue but less fibrosis (which happens added rads) long term physio should get you back in the 90% normal range) but I also found lymphatic massage (only after you've had a clean scan) helped with swelling, and acupuncture to help return blood flow to the area will soften it up.

try to keep your spirits up... you can get through this. hugs