This is an interesting question, but an unfortunate problem to have dealt with for so long. It is always a good idea to talk to a physician in person if you have pain that persists for a long period of time, and especially if you develop other symptoms or the pain starts to become worse. The fact that you have had these symptoms for so long, and that the symptoms seem to be staying about the same throughout the duration of that time is a good sign that it is not likely to be something terribly serious, but it is still important to speak with a physician.
There are many things that could be causing your pain. The area right behind your ear is called your mastoid bone, and when people get ear infections, that bone often becomes involved in the inflammatory reaction as well. Sometimes, pustular fluid can remain in the mastoid long after the ear infection
has resolved. Alternatively, as the mastoid has air cells in it (small pockets of air inside of the bone itself, which usually drain somewhat like your nasal sinuses) these pockets can become filled with fluid and can be sensitive to changes such as upper respiratory tract infections. Other issues such as a headache
, musculoskeletal pain, or a remote injury
could also be playing a part, but it would be impossible to say which for sure without a physical examination.