You most likely hit the superior portion of the femoral artery that runs just below the spine.Two years ago I shot a BB at 35 yards and the angle/distance he was standing at made my arrow cut that vein in half as it passed through him. Anyways, I shot him and knew I hit him good, but was absolutely blown away when he literally took two leaps (about 10 yds) and dropped dead. He literally died in less than 2 seconds. When I gutted him, I discovered I indeed had hit the artery below the spine as my arrow passed through him and there was an unbelievable amount of blood in his chest because it all poured out of the vein into the chest.

Heres a pic where you can see the entrance hole. Its not as far back as what I think you are describing, but the artery runs the length of the spine until it reaches the back legs.