When i run for a long time my upper right pack muscle "under my right chest muscle" it starts to hurt, is it because i don't breath right or what?

Docs can only Guess: Pain in a muscle probably relates to use & fact that all muscles, during contraction, block their own blood flow. With more contraction than relaxation, muscle may become sore. Remember: mds did not design or engineer human beings & are taught basics of partially understood body functions with primary emphasis on signs, symptoms & theories of advanced disease (health excellence never mentioned).