It is not a sign of anything necessarily, if they have done an MRi and found nothing, that means that they have ruled out anything serious!!!
Trouble is with headaches sometimes is that you become convinced that you have something seriously wrong cos the pain is so frequent and intense!!! But the truth of the matter is that if it is something serious usually headache is the last symptom you get, you usually know there is something really wrong by the time the headaches start!!
Try not to worry and get yourself to a Headache Specialist.
Co-Moderator of the Migraine/Headache Forum