Hi
Welcome to the MedHelp forum!
Either a boil or abscess is developing or you have strained the neck muscles by overuse as in computer use. It can also be due to pinched cervical spinal nerves.
Take care!
Without trauma, my guess is a sting or a bite. Because of where this is, the size, and it being sensitive to the touch, I would say to make an appointment with a doctor. I hope the best for you.