Your symptoms are consistent with a low back injury pressing on nerve roots from your spinal cord. The most likely cause is a disc injury in your lumbar spine. Vertebral discs act like shock absorbers between the bones of your spine. Over time the discs begin to degenerate which can lead to pain locally or radiating pain if the injured disc presses on the spinal cord or presses on the nerve roots that come from the spinal cord. It sounds as if you have a bulgin or herniated disc that is pressing on the nerves that innervate your leg (the primary nerve is called the sciatic nerve). When this nerve is aggravated you may experience pain that travels down your leg.
All of your symptoms are consistent with a disc injury. I suggest you contact an orthopedist for further evaluation and treatment options. In the mean time, avoid bending, twisting and lifting objects. Do not exercise if this makes your symptoms worse. You may try resting on your back with a pillow under your knees. If you have difficulty or changes in bowel function or urination (uncontrolled bowel movements, urinary incontinence, feelings of having to urinate or have a bowel movement constantly) go to the ER as this is a medical emergency.
You can find more information about disc injuries here: http://orthopedics.about.com/cs/herniateddisk/a/ruptureddisk.htm
I hope you find this information helpful.
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