Welcome to Healing Well. Sorry to hear that you are suffering with all this.
First of all please do not worry, headache is usually the last symptom for brain tumours, and having further scans will be quite safe, please don't trouble yourself about this.
You need to go back to your GP and get yourself referred to a Neurologist who is a Headache Specialist. This may take some time and you may have to travel to see a good one, but it will be well worth it!!
Most Neurologists do not know very much about headache, so one who specialises in headache will get to the bottom of this for you. As there may be a wait, the sooner you get the ball rolling the better!
Please let me know how you get on??
P.S. Are you in the UK???
Co-Moderator of the Migraine/Headache Forum
DX: Hemicrania Continua, Chronic Migraine, IBS, Allergic Rhinitis and Heart Murmur
Meds: Imigran Injections, High Flow Oxygen, Mebeverine, Lansoprazole, Nasonex and Clarityn