How old are you? I'm 53 and have been living with lower back pain and hips and knees for years but after I ride about 5 miles or so I get limbered up and feel great while I'm riding, now the morning after is a bit difficult but after a few minutes I get limbered up. If it gets to bad go see the sawbones. If you are just starting to ride it could be just a getting in shape thing. Try different adjustments on your seat maybe its too far forward or too far back also the handle bars may be a little out of whack for you, thats the nice thing about the newer bikes everything is adjustable. Maybe you want to consider some lower back exercises to strengthen that area and believe it or not if your abs aren't in good condition that can contribute to lower back trouble. Like I said if it doesn't get better go see the doc. Don't risk permanent damage. Have fun and no pain thats what its all about.