I have three herniated, torn, and dessicated discs in my low back and have been dealing for several years. For awhile I had sciatica too (pinched nerve). This is what my experience has been:
if it's muscular, heat makes it feel better. If it's a disc problem, ice makes it feel better.
A muscular issue should resolve itself in about 6 weeks.
Any shooting pains down the leg, see a doctor asap (you probably have a pinched nerve that needs checking). Shooting pains sideways (radiating horizontally from the spine) usually is a sure-fire ringer for a muscle pull.
If it's an achy, deep down feeling, lay off the step for awhile and any weight lifting that loads your low back. If it feels like your back is "sore" and "straining", that is probably muscular. Take anti-inflamatories and try heat. Those ThermaCare heat wraps are just amazing. Also, strongly consider "core stabilization" work 2-3 times a week, such as Pilates or Yoga for the low back.
THere are a billion reasons for low back pain and 80% of people over 30 experience it at one point or another. As others have said, watch your form when lifting weights (esp bent over stuff and lunges and clean and jerks. If you sit at your job, try to get up once an hour and stretch out your low back.
If you are still hurting in 6 weeks, make an appointment to see your doctor for sure. I sure hope you feel better.