What could this be? (a brain tumor)?

My advise is go see your doctor. It sounds as if you could have High blood pressure. Normal B/P reading is 120 over 80 140 over 90 is considered to be boarder-line hypertension any higher is not good. Dizziness while standing is a symptom of high blood pressure They call hypertension the silent killer, hardening of the arteries, stroke etc. Being that you are only 12 years old diet and exercise and good healthy diet may be the cure. If that does not do it a daily pill will cure it, but is nothing to play with, should be monitored and treated. I do not think you have a tumor, sounds like simply a benign cyst to me. I have had two of them removed myself, one on the base of my neck and one on my ribcage. Just a collection of fatty cells, if it is soft and moveable, more than likely that is all it is. People live with them their entire lives, some like myself choose to have them removed, 9 times out of 10 they do grow back though. Mine did took about 12 years but the darn thing grew back in the same exact spot ! No big deal it was done outpatient, with local anesthesia and a few cuts. No big deal at all. Good luck to you, sure you will be fine, just have that blood pressure checked that could be a terrible outcome if left untreated for anyone not just yourself.