I just finished riding 27 miles and the last 5 miles my lower back (the small of the back) was really hurting. I did stop and take a 3 minute break at the half-way point, and I felt pretty good. My saddle is level so that shouldn't be the culprit, right?

I try to stretch my arms on the bike and I change hand positions frequently. I stand in the saddle and my butt was ok, but my poor back......owie!

Since I'm relatively new to the distance riding, is this normal?