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You probably have an infected lymph node
which can cause this among a range of possible causes of this problem. Especially if you have fever it could be due to infection. There may be infection in the mouth, gums
which can cause lymph node infection. Avoid bad posture
at work, use good neck support, avoid wearing tight ties on neck, get enough sleep with proper posture as well. There can also be possible nerve irritation which can cause this problem. Better to get investigated by Surgeon for proper diagnosis and exact treatment.