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The symptoms you describe are suggestive of a problem in the area of
distribution of the sciatic nerve
In particular I am impressed by the fact that the pain goes down the back
of your leg and the fact that you have numbnes on the sole of your foot ,
which is supplied by the first sacral root of the sciatic nerve.
This would imply to me that your problem is more likely to be related to
a problem in the lower spine than in the leg, which fits with the failure to resolve by cahnging the chair ets.
I would suggest that this need to be investigated by means of an EMG or an
MRi scan of the spine, either of which can be obtained by conbsulting a
neurologist.
At this point you really should have some testing done rather than simply
treating the symptoms with OTC medications.