What Causes Bruising And Tenderness In The Last Two Ribs?

I have been experiencing tenderness and increasing moderate pain on my left flank (side) down near my last two or three ribs for about four to six months now, and getting more noticeable in the last week or two. When I roll over in bed to that side, it hurts enough to make me wince a little now. I have not fallen or injured myself in any way (that I know of) and its not shoveling snow, as my spouse does that. It s a very localized spot/area and feels sort of like a regular bruise and tender to the touch, with no discoloration or swelling at all. What the hell could this be? I am generally pretty healthy and haven t had even a head cold in more than 5 years. Any help would be great....... Thank you.