I'm Curious To Know Why, When I Do The Least

I'm curious to know why, when I do the least bit of stretching, such as reaching to scratch an itch in my side, I get severe muscle cramps on the left side of my chest. These are muscle cramps, I have no doubt about that, for they're just the same as the frequent cramps I get in my legs, though turning my toes up obviously isn't going to ease them as it does the leg cramps. I've been diagnosed with heart failure and unstable angina, I've had one (mild) heart attack and I've had a by-pass operation. I'm very familiar with heart pains and these are not the same. These are definitely muscle cramps. But what could be causing them?