I had sciatic leg pain which I got rid of. But now whenever I lift some weight, I have pain in my lower back for days. How can I treat that?
You'll want to see a doctor, and hopefully get a prescription to be evaluated by a physical therapist, who can assess your body and muscles to identify the cause.
If you can find a physical therapist experienced in myofascial therapy, in my experience, they're extremely good at identifying the real underlying problem. I had sciatica for years, and saw numerous doctors, and several physical therapists, and had no improvement for a decade, and often worsened. A myofascial therapist resolved my pain in 6 weeks. It took longer to overcome other chronic pain, because I had a lot of muscles that over or under compensated for alignment problems.
Myofacial therapists are so awesome! There aren't many around yet, and a lot of doctors are unaware of them, assuming all PTs are the same, so you may want to look for one in your area on your own, at least for an assessment.