Cyst, abscess? This is a common area for an ingrown hair or sweat gland which can get inflammed, swollen, and irritated. Rarely it could also be an infection. See your doctor it may be a cyst that needs to be removed or an infection that needs to be draind. In the meantime, use warm compresses and try to avoid squeezing or poking at it.
Boil. A painful lump on the back f the neck could be an inflammed cyst or abscess. It is important to see a doctor for evaluation and possible drainage. The doctor will probably also prescribe a course of antibiotics. If it is an infected cyst, the course of antibiotics needs to be complete before it can be removed.
Lump on neck. Lumps on necks can be benign lipomas (fatty growths) infections such as abscesses and cysts. There can be other growths that are serious so please see a dermatologist.
Cyst or skin cancer. Could be an epidermal inclusion cyst (sebaceous cyst). These can arise suddenly due to infection or inflammation. They can go away on their own or require treatment with oral antibiotics, incision and drainage, steroid injections, or surgical excision. But you need to have it examined by a dermatologist to make sure it's not a skin cancer which can also present as an itchy painful lump.