I tend to get lower back pain early in the season, when I'm untrained, or try to take on too many hills. It goes away the second I get off the bike. So some of it may just be conditioning and getting used to riding more and longer mileage. It's pretty normal to have some discomfort if you go from an 8 mile ride to a 36 mile ride!
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow