Don't worry about it. Being able to draw boxes from different angles and rotations will become a lot easier the deeper you get into these lessons and the rotated box exercise is sort of meant to be beyond most beginners abilities to get them to try something they aren't comfortable with.

For your shoulder a little soreness was happening for me at first as well but not that bad. I would definitely practice for less amount of time than you were previously doing until your muscles get used to being used more.