I walked home from the store, something like 3-400m, carrying a bag that could weigh somewhere around 15kg. I carried it with my left hand, and when I got home and started unpacking my arm started to shake violently (could not for the life of me keep it still). This has never happened to me before, and I have a physically demanding work so this kind of weights are nothing new to me. Either way, I´m scared to death a bout this now, and even though it has settled down a bit the whole arm is still shaky and aching.
Any ideas what this could indicate?