My head feels weird when I shake it fast, almost like my brain is moving inside my head from the momentum. What causes this?

Physics. Head moves, brain moves. Its really not that hard..

Its your brain moving inside your head from the momentum

Man: 'Hey doc - it hurts when I do this...'

Doctor: 'Well don't do that!'

Just don't do it all the time. With those g-forces at work, it one day can cause too much water like substances to develop in your brain bag or not, and cause all sorts of problems.

Shaking your head is causing it. If there's no pain, you should be fine but you can always see a doctor if you want.

Do you feel pain when you do it? If so you may be hung-over