I'm no Cathe, but hope this helps!
It could be your form...you have to have precise form to orchestrate a pain free deadlift. Are your knees locked or slightly bent? is your back flat or is it rounding? Are your sticking your tail bone out or keeping it tucked under causing your back to round? Do you have the weight too far out if front of you or is it grazing the edge of your body? Are your shoulders retracted or are they rounding forward? All of these are crucial to performing a good pain-free deadlift. Make sure: you keep your knees soft or even bent slightly if you have tight hamstrings; you stick your tailbone out as you go down helping maintain a flat back and keep your chest up and shoulders retracted - think of it as if you had a metal rod running thru your spine and you can't bend or break that metal rod; make sure as you go down the weight is staying close to your legs and not way out in front you, and make sure the weight is not too heavy. Maybe until you really perfect your form, use lighter weight so you can get your form feeling natural. And make sure you engage the glutes and the hamstrings at the bottom of the movement pushing the weight down through your heels as you bring the weight back up, standing all the way up nice and tall. If executed like this, you should have pain in your low back. If you have trouble with this you may find that the more you bend your knees the more apt you are to stick your tailbone out. Think of pointing your tailbone toward the wall behind you and feel like you are really sticking your bum out!
Beyond that, if it's not your form and you are still feeling pain even with perfect form, I would start focusing on traditional squats more. They are one of the best exercises you can do not only for the legs, but also for the core and low back. A good way to engage the low back and core when doing them is doing "prisoner squats" with the hands interlaced behind the head. Squat through your heels and again drive the tailbone back, like you are trying to find a chair behind you.
Hope that helps!
