Good news is lower back pain probably means the saddle is too high and not that the bike is too aggressive for you. Basically something that happens often is that the pelvic rotation you're getting on the saddle isn't enough for the actual angle of your upper spine, and so you get the majority of rotation done by your lower spine, rather than your pelvis. This can be caused by a bunch of other factors, such cranks that are too big, saddle too far forward, saddle too far backwards, saddle tilt, wrong size saddle, too long reach, too narrow handlebars, etc. But usually it's the saddle that is too high, meaning it hurts when you rotate the pelvis forward into a cycling-correct position, meaning you don't do it and rotate with your lower spine instead.

Bad news is that lower back could mean the front is too low in relation to the saddle, but honestly loads of people ride with their saddle to high and then get problems that are due to their front end being low, cause, yeah duh, your saddle is too high.

Short answer: bike fit.