Neck pain, major headache, pain back of head, nausea, tachycardia, blurry vision. had brain mri which was clear. if not tumour what could it be?

See a neurologist: Did you get brain vessel imaging? There are many types of headaches, including migraines, cough, cluster, occipital neuralgia, muscle strain, etc. Keep a log of the timing, duration, severity, and location of your headaches. Also try to identify any headache "triggers" (stress, not sleeping/eating). Take this information to a neurologist. http://www.nlm.nih.gov/medlineplus/headache.html