RELAX...Don't Do It

3rd class of Aikido since my break. On the first night I think I hurt, if not tore, some abdominal muscle. On Monday I discovered that when I did a forward roll it hurt like heck across a line in the area a few inches below my navel. BUT if I really relaxed my abdomen it was totally OK. I had the same problem tonight. So when like a novice I tensed up on a back roll, I felt the pain. And it was only when I paradoxically forced myself to relax when I was tense...I really did relax. And it mostly worked.