Form is first - have you had anyone ever really instruct you in dead lifts? They can be VERY damaging to the lower back.
Secondly, when you're lifting, you always want your back to be in lordosis - that's when your lower back is arched inwards. Then you tighten your stomach to create intraabdominal pressure. That region inside your body is 95%+ fluid and doesn't compress easily - so you create this tight "fluid ball" in your stomach region by arching your back and tightening your stomach that supports your spine.
That's why some people wear weight belts -- it's to create this force on the stomach region to tighten the fluid ball, and support your back. (Of course, you don't want to train regularly this way, because then you're a wimp without the belt since you didn't train the appropriate muscles).
Thirdly, lower back pain is something to take very seriously as the entire spinal column is very difficult to heal. Before you start lifting like that again, make sure you get some help from someone who is experienced, and who can critique your form. Ideally, you're putting no real pressure on your lower back (since it's supported by the "fluid ball") but if you don't have proper form, this obviously isn't the case.