Not sure but I would think its growing pains. Two nights ago my almost 4 yr old woke up crying saying his leg hurt. I asked where and he said in his bone! My 2 yr old shares a room with him and she will often copy him to no end, drives me nuts, could your 2yr old be copying too? If not then keep an eye on how often it occurs and see what a doctor or naturopath reccomends for kids! Mine was random and hasn't happened again so I'm thinking some growing pains!