How tense is too tense?

Or should that be: how stress is too stress? The only reason why I bring this up is that today when I was trying to take off a shirt I pulled something around my right shoulder blade. It hurt like mad! I mean it seriously hurt! I could not do anything to relieve it. After listening to my teacher lecture about warm up and stuff and trying to read for my class, I learned something: that you must warm up or get injured. So how does that relate to me? I will never know. I look forward to when I can get a massage. That will be so nice and relaxing.