I know that aches and pains in your back and other joints can happen quite early on, as the progesterone hormone (which starts to increase from conception) has the effect of making your ligaments more stretchy, leaving joints less supported than normal.
Everyone's sensitivity to progesterone is different, and and if you've had babies before then your tissues will probably be more stretched than somebody who hasn't. So you might find that the aches and pains start earlier this time round than during your last pregnancy.
Saying that, if the lower back pain is very severe you might just want to get checked out by your doctor or midwife for your own peace of mind.