Today I was stretching my right hamstring. A standard stretch, one leg tucked in and the other extended, then lean forward. As I brought my face down to my knee, the upper right side of my abdomen tightened up and became hard. It was very uncomfortable, and fairly hard. It went away after a few seconds, but when I stretch my abs that area feels sore. This has happened about three times now, the last was a year ago. Any thoughts on what this is?