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The low back pain
which is causing some weakness in legs can be due to prolapse intervertebral disc
(slip disc).You need to consult a Orthopaedic doctor to get examined & for treatment.You may require investigations like X-ray of spine and if necessary MRI
of spine to locate the problem if you don't get relief with medicines.Since you said you cannot afford if the treatment is expensive then talk to the doctor & get treated at some government hospital.