There are many causes of lumps in the back of neck. The most frequently seen lumps or swellings are enlarged lymph nodes, which can be caused by bacterial or viral infections, malignancy, or other rare causes. The lymph nodes in the back of neck are the occipital lymph nodes, the posterior cervical lymph nodes and the jugular chain of lymph nodes.They can swell up after infection or allergic reaction.
Lumps in the skin or just below the skin are often caused by cysts, including sebaceous cysts.Sebaceous cyst is a closed sac or cyst below the surface of the skin that has a lining that resembles the uppermost part (infundibulum) of a hair follicle and fills with a fatty white, semi-solid material called sebum produced by sebaceous glands of the epidermis. http://en.wikipedia.org/wiki/Sebaceous_cyst
It can also be a swollen muscle.
My suggestion would be to wait it out for a few weeks to see if it changes in size. If it gets smaller, it was nothing. If it happens to get bigger or is associated with any other features, then make an appointment with your doctor.You may take some pain killers or some anti-inflammatory drug if you have pain.Application of warm compresses may also help.
Hope it helps.Take care and regards.