I am having a problem with the right side only of my lower back after about 45 miles or so. It seems to be fatigue related. I have had my bike fitted and have messed around with the saddle, stem, bars, and such. The bike guy says that my bike is just a tiny bit to big for me, so I am a little stretched out on it, even with the stem as short as it can go...I have heard this before...usually followed by, "but only a little bit...you will probably be ok." The saddle can go a bit further forward, but then it jacks up my leg position in relation to the pedals. I love, love, love my bike (Colnago C50, full Campy Chorus 11) and really don't want to start at ground zero again with a new one, but I also ride with an aggressive bunch, and I need to be able to hammer for 100 miles without killing my back. It is on the right side only (weirdly enough, and yes, I have checked my saddle to make sure it is even, and checked my hand position, etc.) right about where the waistband of my shorts would lie (I wear bibs usually).
It is worse at the moment because I am out of shape, when I am in shape it really doesn't kick in till mile 45. Right now it is starting at 30 miles. It doesn't bother me at any other time. Anyone? Anyone?