possible tension headache?

Hello. I'm a 26 year old healthy female with no history of sickness. I went on a vacation in May and when I returned, I became ill. I was nauseous for about a week. After that the headaches started. I had a CT Scan in June which was negative. The pain/pressure is always in the lower back end of my skull, above my neck. Sometimes I feel a warm, sort of burning sensation in the right side (back) also as well a pulsating feeling through just the back part of my skull. This headache can last anywhere from 1-12 days, and it's usually constantly throughout the day and night. Mornings are the best, I wake up and am usually ok til the afternoon, nights are the worst but so far, they haven't prevented me from living out my life. I began seeing a Neuro about a month ago when the frequency of these headaches increased. I was given a few different Migraine medicines (Maxalt for example) and nothing seemed to really work, yes they did ease the pain but the pain always comes back. Today I was given Amrix which is a muscle relaxor because I do feel a lot of tightness in the back of my head. I took one about an hour ago and the pain is still there. I am getting an MRI next week. I'm a little scared. If the CT Scan was negative 6 months ago, do you think within 6 months I could have developed a tumor or anuryesm? I don't know what else to do to get rid of this pain.

Thank you.
ps: I am on a birth control pill that I've been on for years, also taking a One-A-Day Vitamin, a B12 and a Magnesium Vitamin too