Sounds like textbook BPPV and yes I have had it too. My nightmare started last August and I am getting somewhat better. I have had the manipulations with my head by a Neurologist, etc. to NO avail. It is better but no where near completely gone.
There are surgical options for this, but used as a last resort. I would keep trying the conservative measures.
I had vertigo for two years. There are stones in your inner ear and they have fallen out of place and need to be put back where they belong by having your head moved around in a circular motion. I dont know how you feel about chiropractic care, but I went and they didnt have to do an adjustment. I have now been virtigo free for 3-4 years now! I dont know where you live, but find a good Neurological Chiropractor in your area. Good luck and keep me posted.
Sometimes the basic Epley manuever doesn't cure BPPV; sometimes you have to repeat it or else see a vestibular physical therapist who specializes in dizzy patients. There are different maneuvers depending on which semicircular canal the debris is in; a vestibular PT will know how to do these.
So those are my suggestions (neurologist specializing in headache or dizziness; and/or a vestibular physical therapist, who sees mainly or exclusively dizzy patients). Check out the Web site of the Vestibular Disorders Association for help finding specialists.
Brain tumor is extremely unlikely, so please don't worry about that.