yea, firstly, don't do swings yet.
Secondly, my back would become tight and irritated when I first used KB's because I was using them wrong, almost everyone has this problem. If you can find a kettlebll certified person in your area then definetly meet with them for a half hour.
Firstly, the swing looks like it comes from your shoulder. It shouldn't. All the movement should come from your hips. Start out with your feet somewhat wide than shoulder width. Have both hands on the horns of the bell. Bend you knees so that they are almost at 90 degrees. Keep your back arched and always looking forward. You should be holding the bell between your legs and slightly behind your knees. Now Nudge the bell backwards like you are hiking a football and explode through your hips and let the bell pop forward. It should come up about to your waist. You will be standing up straight, with your arms locked out and the bell at waist height, your legs lock and you butt muscles clenched. Now, fall with the bell. Like when someone tosses you a bowling ball and you kinda go down with it to absorbe the shock. Bend your knees and bring the bell between your legs again and let it go back far enough so you feel some tension on your hamstrings. Now, explode through your hips again. You should barely feel it in your shoulders, its all in the hips. Exhale on the way up, inhale on the way down. Practice just your swings for two weeks and then your ready for more.
It's not uncommon at all that your back hurts. It will hurt if you do it wrong, however I find that if my lower back gets a little tweaked doing deadlifts or any other lift, some good kbell swings help loosen it up.
check out punchgym.com and watch the movie trailer for art of strenght. you will breifly see perfect swing form. when you have perfect form you can do 100 swings no problem. also, buy the DVD if you got the cash, best workout with a kbell ever!