I Woke Up In The Middle Of The Night To

I woke up in the middle of the night to the feeling that my left arm wasn't quite right. It felt twitchy and slightly achey, and when i try to flex it, it jumps and flexes really fast. I have no control over the speed. What is this? It feels awful. I hadn't been laying on it because I'd been restless and laying on my back most of the night.