So I recently purchased a new bike and today while riding I experienced lower back pain. I did not have this problem on my first ride this Saturday. When I got home I whipped out the tape measure and checked the length from seatpost to stem and the seat tube length and compared them to my hardtail which I've ridden without problem for years. Both of these measurements are more or less the same.

I'm 5'10 and I'm riding a medium Stumpjumper which is about the same size as my 17.5 Trek 4500. Any theories about why the back pain? I'm thinking maybe I had the seatpost too high?

PS. I'm 28 with no history of backpain.