I'm thinking psoas, too. If there's a big ol' shortening of the psoas (which I guess could come from a strain biking or swimming) it can form a knot, which has felt pretty superficial and anterior on some of my patients. Sometimes it can get tight enough that a patient can "feel" it when they lie on their stomach on a hard floor.

But tender to the touch isn't something I've ever encountered. Could rectus abdominus do that? Aorta? (naw, wrong spot...)