Shoulder pain after running / how can i stop this

[RichPiHAHA]

For the past 3/4 months of running daily/weekly i've noticed that sometimes i get pain on the top of my shoulders when i finish, i never researched into this because i assumed it was just tight muscles that would loosen up the more i ran.... They haven't i just now finished a 2 mile run and had terrible pain on my shoulders when i was done, so i googled it. Apparently this comes from bad posture and sitting at a desk too long, i'm a student and i defiantly sit at the desk a lot, my question is can this be fixed simply by changing how i sit in my chair every day? or is this permanent damage, if i can change my posture while i'm young now and prevent shoulder pain when i'm older i'd love to do that.