Lots of injuries can cause back pain like this. Ligament strains, muscle tears, even cracks in the bones of the back. Obviously you should stop doing the lifts that aggravate this for a few weeks and concentrate on other parts of the body and some aerobics for a while. Another type of injury is a ruptured disc. A ruptured disc can cause back pain but can also cause pain radiating down the leg, especially when you hold the leg straight out in front of you.
My back hurts right now on the left. It is annoying and riding in the car makes me nauseated, even though it's more of a discomfort than severe pain. Mine is in one of the dimples down by my sacrum. I believe if the pain is there it means a particular muscle is pulled, so that's good news. Check and see if you can put your thumb where the pain is and it's in a dimple in your back, sort of midway between the spine and the outside of your hip, several inches below your waist.
Most back pain goes away on its own in 6 weeks. Reduce your activities, but don't stop walking. Walking is the best thing you can do for your back. Try sleeping in different positions and see if it helps. Try soaking in a warm tub with Epsom salts.
When it feels better and you start back, go slow and stop if it hurts again.