What could a small lump be on the back of the neck and ear?
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 had one and I was 28. I would get it checked out by an Ear nose and throat doctor for peace of mind. It could be a bone spur.