Masturbation

I've been having a problem with masturbation ever since I found out what sex was. I tried it when I first found out, but it didn't set off any "magical experience" or "fireworks" simply because I didn't reach my climax. Now that I'm older, I know some of the tricks and tips for it, but whenever it starts to feel good, my arm will cramp up if I'm using a toy, and I move a lot when using my clit which makes it impossible to finish. I've tried doing them both at the same time, and it worked halfway through. But I moved around a lot and my arm cramped and it just sucked. Am I doing anything wrong? Like, should my arm ALWAYS cramp up when using a toy?