Feel the Hate

How is it that I can fight rapier Saturday, Sunday and Tuesday with no ill effects except some residual stiffness, but I bend down to get something out of my closet yesterday morning and end up with a massive knot in my back that is still causing me a fair amount of pain today? Don't know if it's a pinched nerve or what, but it's going to make fighting tomorrow a real problem if it doesn't unkink by then. I'm utterly convinced that my body hates me and does this kind of thing just out of spite.