Twiggy,
Why do you think CoreMax is responsible for your pulled muscle? Are you pulling on your head rather than using your abdominal muscles? What Cathe HC workout did you do the first time you pulled your muscle? Was it CoreMax as well or another one? Hope I'm not asking too many questions, I'm just wondering what specific workouts you're doing and the exercises they contain that may be causing this.
You mentioned that you fractured your left shoulder a couple years ago. Could the pain be from pushing or pulling too hard on that side during shoulder exercises? Can you feel the pain right when it happens while working out, or not until afterwards?
Hope I haven't scared you away and you go back to lurkdome.
I'm just trying to figure out what exercise may be the cause of your pain.