Does anyone know what this weird sensation in my head could be?

I suffer from anxiety and I’m definitely a hypochondriac.
For quite a few years now on and off I get this weird rush in my head. It feels like a rush of adrenalin. I can just be sitting doing nothing and I“ll get it, It’s the same sensation you get when you stand up to fast but I don’t need to do that to get it.
Does anyone else have this – lately it’s been quite constant, I also have been having quite bad anxiety lately and I’m wondering if maybe it could be a brain tumor?
Thanks
I have been to the Dr’s but they just dismiss it.