Zoom-zoom,
You may not have back issues now, but this may be an early sign. A quick consult with a physical therapist may show the answer.
I used to run, but quick because of back problems. My chiro recommended switching to another sport, and that's how I found cycling. His X-rays showed a twisted and tilted pelvis that results in lower back pain and in the extreme, sciatica.
A second episode put me in the emergency room. One of the top orthopedic Dr's in our area found a bulged disc in an MRI, but "it shouldn't cause you that much pain."
I demanded a referral to a physical therapist. She was my god-send. She also diagnosed the pelvis twist and tilt (I didn't share my chiro's diagnosis until after). She taught me the stretches I would need to do twice-a-day for "the rest of my life, unless I wanted to come back and visit."
Apparently leg-length issues are frequently misdiagnosed--I thought one leg was shorter than the other, but it was actually one hip higher than the other 
I had noticed recently my Brooks saddle was more worn on one SIDE than the other. DH blew it off. Now I notice the whole seat is starting to tilt to the left--it's two years old. Fascinating. I would post a pic, but it's subtle and won't show in a pic.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17