Why is my arm twitching?

My upper right arm muscle has had this on/off uncontrollable twitching all day long. It doesn't hurt, but it is just super annoying. It seems harmless, but I have no idea what it is, so there's no way to tell. If anyone knows anything about this thing, or has had it before, let me know. I'm mostly curious about what it is, but if anybody has anyway to get rid of it, please share that as well! Thanks!