It's probably a cyst or possibly an ingrown hair. Seldom does cancer strike the back of the neck. It's best to see your doctor and let them decide if the cyst, etc., should be removed or if it's simply an ingrown hair.
A lymph node.
Sometimes (thou rarely) a lump behind the lower part of the ear can be a cyst or a tumor of the parotid gland. Again this is rare and shows up mostly in men above the age of 50, but...
i am 11 years old and two days ago i found a small bump on the left side of the back of my neck. it kinda hurts and i am very concerned. i asked my parents and they don't know( they are both great...