Low back pain can be due to muscle pull/strain. However there are other causes too. Pressure on the lumbar nerves can cause pain due to spondylotic changes in vertebrae, degenerative bone disease, herniated disc, bone disease, cancer or TB of the vertebra or bad posture. Kidney problems, stones in the urinary tract and colitis or IBS are the other possibilities.
There is a chance that you have compression of the spinal nerves in the lumbo-sacral spine region. This can happen due to overuse of the lower back as in work requiring too much of bending, lifting weight, fall on the back, overweight, canal stenosis, bone disease, spondylosis, poor posture etc. Poor posture while sleeping, sitting on computers or jobs for a long time, uneven bed, exposure of back to cold air while sleeping can cause a discomfort for a few days. Take a multivitamin, plenty of fluids, and an OTC analgesic. Apply a local analgesic and apply a heating pad and see if it helps. If this does not help, consult a neurologist. A MRI of the spine and nerve conduction studies may also be needed. The treatment is to remove the compression. You have to discuss the best treatment option with a neurologist, which can range from medication to physiotherapy to traction, lumbar belts, lumbar corsets or even surgery. The best thing would be to consult a general practitioner or PCP and try and assess what is the cause of your problem. Then proper referral for concerned specialist can take things further and get you proper treatment. Take care!