Every time I do parkour, the only thing that really hurts is my back. It seems like every part of it gets pulled. The muscles under my shoulders and around my shoulder blades are the worst. I was wondering if this happens/happened to any of you. I was also wondering if you could tell me some stretches and workouts for the back because I only have a few of them.

Got any pictures of your upper torso I can look at to see if there's imbalances  (Cheetaur is a good person to ask too).

Generally, shoulder pain and around the scapulae have to do with multiple reasons a lot of which are strength and posture related.