Hypotension

Is then when you get head spin and really dizzy when standing up?
I get this quite often after a heavy session and thought it was low blood pressure

Yup, the same thing.

always thought that was under-sugar or "bonking"?

I borrowed a testing machine and measured my blood sugar after having a dizzy spell, and it was 'normal'. Bonking for me feels quite different, I normally feel very nauseus which is something I don't get with Hypotension.