What Causes Muscle Spasm At Sternum?

I get a muscle spasm at my sternum on the sides when I do abdominal crunches or contract those muscle when I bend over to reach something. I feels just like a charlie horse and I have to stretch to relieve it. I can't tell if it is the upper abdominal muscles or the intercostals where they attach to the sternum. What would cause and how do I prevent it? I do excercise regularly, about 5 times per week