Tensing up during orgasm

Okay so I have had an orgasm before but only a few times because everytime I’ve gotten close my muscles tense up and it’s like i have no control my body tries to jerk away or my legs won’t relax I just want to know if this is normal and if so how do I learn to relax and allow it to happen....I just want to enjoy an orgasm without my body fighting me against it😓