Feeling of blood rushing. Clogged?

I occasionally feel, feeling of blood rushing through my head when i exert myself , from rest position suddenly, i have felt it when, i am suddenly run after sitting idle or i am relieving myself in toilet(constipation) or getting excessive angry or while i finish off during sex, it does not happen every time but it happens like every 4th time id say, or if am really overstressed the feeling how do i tell is like feeling blood flowing heavily through your head and feeling heavyness in head, feel no pain though, and it subsidize on its own within second, but it scares me im just 21 , should i get mri head done, and what test should i have zone before consulting a doctor, i have high cholesterol, is my brain arteries clogged? , i once had high bp 133/86 but next day it was 128/72 is it due to high bp, and i do feel pulse in my butt and fist if i concentrate on heart rate is this related wht should i do.