if your Standard Deviation is 15.5/100 hands your "true" winrate is somewhere between -1.36 and +1.36 Bb/100 (95% confidence interval).
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This downswing could be statistical. it most likely is.
I cant imagine someone with those stats and a solid postflop play being a loser in the 3/6 games.
You will not be a huge winner with such tight preflop standards, but you will probably not be a losing player either.