My headmaster’s cane was perhaps not so much thin and swishy as thick (between a centimeter and half an inch, if you’ll forgive the mixed units), although still fairly flexible as he demonstrated by bending it through about 15 or 20 degrees in front of his chest.
I imagine your black shiny football shorts afforded about the same, neglible, protection as my white cotton PE ones.
You say this was your last caning, whereas mine was my first. This raises the question of whether you were in fact a more experienced canee, as it seems to be suggested that this ‘numbing’ or ‘anaesthesia’ might be a phenomenon which affected the inexperienced.
Another point is whether you had been told you were to get six and might therefore have been deliberately counting each one.