It's completely normal, in the best way I can explain it your body tenses up and the shaking is just a process of that when you masturbate.
As quoted " the human body goes through when you have sex or you masturabte... Pupils dilate, arteries constrict, core temperature rises, heart races, blood pressure skyrockets, respiration becomes rapid and shallow, the brain fires bursts of electrical impulses from nowhere to nowhere, and secretions spit out of every gland, and the muscles tense and spasm like you're lifting three times your body weight. It's violent, it's ugly and it's messy, and if God hadn't made it unbelievably fun, the human race would have died out eons ago."
Basically what Billy and Dog said
Your masturbating correctly and reaching an orgasm.
It's happened to me when i first started wanking and reaching orgasm, I then got used to it and no longer shake really.
some people shake when there about to cum and its very normal nothing to worry about
with feelings of euphoria and well-being? Sounds like you're having an orgasm.
Your probably doing something right.