A clinical examination of this swelling is very important to confirm the diagnosis. From your symptoms it seems that it is an enlarged lymph node, a small cyst or growth under the skin in that area.Do you have sore throat, sinusitis, dandruff or any scalp infection? These infections can cause enlarged lymph nodes. Is it freely mobile under the skin? Since you have noticed it now and do not have any other associated symptoms there are chances that it is mostly a benign type of lump. It is best to get it checked by a doctor and get the diagnosis. He will be able to have a look at the lump on your skull and determine what it is and if it is something serious that needs further workup or treatment. Treatment will depend on the cause. I sincerely hope it helps.
Take care and good luck!