A knot at the back of the neck can be related to your incision and may not be related. Since a year has passes since surgery, chances of the lump to be related to the surgery (like localized blood collection, abscess or pus collection) are very less. Yes, cyst is a possibility. It can also be a neurofibroma (collection of nervous and fibrous tissues and a non-cancerous growth). Since it has developed under the surgical incision, your doctor is correct in asking you to show this to your surgeon immediately. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.