It may not just be from the bike... I get the same exact neck pain occasionally after some 80+ mile rides. At times I can do 5 straight 80 mile days with no pain and at times it appears after only one. Once triggered from the long ride it can take a week or two to fully go away. I've found my work desk at home and using the computer I feel the same pain after the long ride triggers it. You may have other factors putting stress in that region and the bike is just one more and enough to take it to the next level.