It sounds like normal tension release that happens during meditation. Aren't there any meditators here? I've been meditating for several years, and my hands, arms and head start to move "automatically" in certain patterns. And I have seen the same thing with other experienced meditators. It's just tension release. Actually it feels really good if you just go with the flow. Don't place too much focus on it. Get your focus back to the mantra.