The 5 most common causes:
1) Muscle Injuries: cause one third of all back pain
2) Degenerative Disc changes. This is a normal part of aging, but often does NOT cause pain
3) Bulging or slipped discs. May cause sciatica.
4) Spinal Stenosis. Thickening of bones or ligaments can irritate nerves
5) Spinal Instability
Strong and healthy core muscles play a big part in preventing and recovering from back pain. Almost everyone tweaks, sprains, or injures their back at some point in their life. Stay ahead of the game by getting regular massage and doing core strengthening exercises.
If you do have sudden severe low back pain, it is often a strain of the Quadratus Lumborum muscle. Massage and patience are two keys to surviving the acute stage of this injury.
Options for core strengthening include yoga, physical therapy, swimming, and strength training. In general, avoid the circuit weight machines because they support your core. You miss the opportunity to balance your whole body. In worst cases they can lead to exaggerated muscle imbalances.
One of my favorite sources for excellent whole body workouts including beginner and advanced difficulty is Jessica Smith TV. https://www.youtube.com/user/jessicasmithtv. She has many free videos on YouTube, as well as programs to purchase.