The various causes of the compression are pes plano valgus (flat foot) whereby whereby it is theorized that the nerve is stretched as the foot turns out with weight bearing, venous insufficiency with congestion or varicosities (enlarged veins) within the tarsal tunnel, and/or scar tissue in the area from perhaps a previous injury. Lumbo-sacral (lower spine) nerve root compression, although unusual given this presentation may be responsible and should be investigated.
An early peripheral neuropathy from most commonly diabetes, perhaps alcoholism, or other more rare neuropathies can initially present with the symptom described. However, given the unilaterally
presentation, this would be atypical.
In terms of treatment, arch supports may be appropriate for the flat footed patient as well as non-steroidal anti-inflammatory medication assuming the patient has no pre-existing allergy to this type of medication, peptic ulcer or gastro-esophageal disease, and/or is taking an anti-coagulation (blood thinner) medication.
However, prior to any treatment, I would recommend an evaluation by a neurologist, or perhaps a primary care physician, to investigate a systemic cause.