Baby shaking while nursing/ falling asleep?

(I'm waiting for the Dr to call me back) - as I was nursing my 6 month old and he was falling asleep, his arms and head started shaking?? Has anyone else ever experienced this? I checked his eyes for potential seizure activity and when I held his hand it stopped the shaking, lasted only a few seconds but definitely freaked me out a little bit. From what I see online this seems to happen sometime, has anyone had a similar experience? I called the Dr and am waiting for a call back while monitoring him while he's asleep but I'm not sure if I should just take him to the ER to be safe or if it's just part of them growing or what??