There are many things that can cause lower back pain, but the most common of these is sciatica. Sciatica occurs when the ‘sciatic’ nerve is trapped or damaged which then causes painful feedback to the brain telling you something is wrong. As the nerves both send and receive signals this can also cause other symptoms that you might notice, such as tingling, lack of movement etc. At the same time, the sciatic nerve being the largest in the body (the one that runs all the way up the spine to the brain), means that damage here can also result in pain and odd sensations elsewhere. Thus you may also experience pain in your legs, buttocks and feet.
If this describes the symptoms you are experiencing then you likely have sciatica. In some cases this will temporary and the nerve will become dislodged or the bone/muscle placing pressure on it will move. In others you will require either surgery or epidural (an injection of steroids into the nerve) to kill the pain, or physiotherapy.
Another potential case of lower back pain (on either side) is IBS or other bowel/intestine problems. IBS is a condition that results in occasional bouts of either diarrhoea or constipation and it is unclear whether this is caused by stress and psychological causes or by underlying bacteria. It seems linked to chronic fatigue syndrome and triggered by stress. Unfortunately there is no cure for IBS, but you can control it by learning to control stress and by eating a high fibre diet.
Kidney stones are another potential cause of lower back pain. This is caused by a solid stone of calcium or another mineral that can cause pain inside the kidneys. As we have one on either side at the back this pain can occur specifically on one side of the body. Fortunately the other symptoms make kidney stones fairly easy to identify. Normally these will also cause pain in the abdominals, pain during urine and potentially blood in the urine too. Doctors will test for kidney stones using a urin analysis or imaging scans. They will then usually use surgery to remove the stones.
As well as a trapped nerve there are many other muscular or joint problems that can affect any point on the back. These can be misalignment of the spine, herniated disks, pulled or twisted muscles, bruising or other trauma and more. These will usually require the help of a doctor or physiotherapist though in some cases they might subside of their own accord or through gentle massage.
Another common cause of lower back pain that is structural is muscle imbalance. Most people think of the muscles as giving us strength and performing a mechanical/functional role in the body to generate movement. However their role is much greater than this and they are also crucial for providing a 'support network' for the joints and organs. They hold the body upright and generally ensure that everything is in the right place – without muscles our skeleton would simply collapse. Normally at any time our joints are under tension whereby they are held upright by equal force on either side. Thus when one muscle is developed to become stronger than another, this can cause an imbalance whereby the bone or joint is pulled to the wrong angle causing pain. Muscle training to return balance to the body is the easiest way to fix such problems.