I have two years of science from university and am currently getting my nursing degree so I have some medical knowledge but I would advise going to the doctor as soon as possible. When you make an appointment say it is urgent and make sure to get a second opinion, don't let them just brush it off. These could be symptoms of many things such as a neurological condition, an eating disorder which would affect heart rate, and could involve blacking out etc., or it is possible that it is connected to your past migrains but that they are getting progressively worse over time. It's true that you couldn't rule out a brain tumor but I wouldn't jump to conclusions to quickly. The symptoms you are describing and the frequency is alarming and you shouldn't let it go any longer and get to the doctor as soon as possible. Not only is there damage done everytime this happens to you and it is stressful on your brain and system, but maybe next time it could be more serious or you'll hit your head hard on something sharp or hard when you black out. I hope all goes well at the doctor and be sure to tell him/her all of your symptoms and if necessary get a second opinion or ask if you should see a specialist. I'll be praying for you and good luck.