It's very normal for this to be the case after a neck dissection. The physiology of a neck dissection is this. The lymphatic system runs parallel to your circulatory, the job of this system is to take your blood and clean it (through the nodes) and then dump it back into the circulatory system.

So if you remove say - 40 nodes- you've removed the pathways for the lymph fluid to travel, therefore it gets stuck in the tissue.

This is why you are swollen, and likely there is a lot more lymph there now because the body's reaction to trauma (surgery) is to send blood and fluid into the area to heal it.

Look into lymphatic massage. A few visits will teach you how to do it to yourself, this will help build new pathways to drain and improve circulation. I'm 2 years out from surgery and most days I am fine. Sometimes I am a bit swollen, often due to how I've slept. Exercise helps too because it helps move your circulation etc...

best of luck!