Abnormal sensations or loss of sensations in a particular area of the lower limb which is associated with low back ache, suggests pathology of the sciatic nerve
or its branches. Usually the pathology originates in the spine most likely disc disease
or some other cause which is putting pressure on the nerve and causing the abnormal sensations on that particular area of nerve distribution. the area involved in your case is S1 dermatome. Some times loss of sensation in perenial area can occur. Better consult a orthopedic doctor and get a MRI
of the lumbo-sacral spine.