Welcome to health care magic.
1.The possible cause could be
lipoma in the first place - painless mass with dull ache.
2.The other possible cause could be infective cause with the collection of the pus with in, followed by a
dermoid in the bone.
3.In this case i would recommend an
ultrasound scan to evaluate the lesion weather solid cystic, muscular or osseous, and where it is extending to.
4.If mass evaluated an FNAC- fine needle aspiration cytology will be helpful to find out cell nature.
5.Dependind of findings a CT If necessary to make sure that there no skull or brain involvement.
6. In case of lipoma followup is needed. Good luck.
Hope i have answered your query.
Any thing to ask do not hesitate. Thank you.