Appendicitis or muscle?

I've had a slight discomfort about 2 or 3 inches below my belly on on my right side for quite a while now...I'm talking a year or more. It's never gotten any worse and it goes away usually after a day or two, but comes back sometimes after really moving around/stretching (playing tennis, running. etc). I can put pressure on it and not feel it, and I only really feel it when I flex my abdominal muscles. (It's actually right along the abdominal muscle line). No loss of appetite, no stomach sickness, no fever or anything. What do you think?