Back itch beginning after 3 years

[Blind Pete]

I bought my spa new three years ago.  I have always maintained my chemicals, ph levels, and all prescribed maintenance.  After 3 years of bliss, I began this year to experience itching only in my back, where the jets hit me.  I changed the water and installed a brand new filter.  But the problem persists.  I understand it to be diagnosed by others as bacteria infection of hair follicles.  But I am doing everything that I always have done, without problems, for three years.  The spa salesman mentioned the ozone injector, but said if so, that I would be having to use more chemicals to keep the water clean.  This is not the case.  I need no more chemicals than I have in the past three years.  I use chlorine.  I keep the spa at 103 degrees.  The chemical balances and ph all test fine.  No other areas of skin are bothered, only my back where the jets hit.  I tried sitting further away from jets, but no improvement.  I am out of ideas.