Is it normal that i shake uncontrollably during sex?

For you to comment you must understand my story fully first. I experimented with a boy when I was very young and since then we do stuff together. We only see each other about once a year so it isn't very often. Nobody knows about it and we dread the thought of someone finding out. He is not related to me by blood so it is not gross. He is my brother's step son. Now to the important part... Everytime we are together and we share a room, I am really eager to have sex and I think he is also but no one mentions it until we start talking about sex, get horny and some way we end up doing it. As soon as we start, I start to really shake hard, my whole body does. It is as if it was extremely cold. I don't know why it happens but it does scare me. I do it anyway and try my best to stop shaking but it doesnt really work. Once I orgasm, thats it, I stop shaking and feel horrible about what happened and dont want to do anything else. I have to be the one to ALWAYS orgasm last otherwise I dont enjoy anything! The reason of my "shakings" might be because I feel that what I am doing is wrong but then again I am gay so it boggles me :S Is this normal??? any advice?? please help...