What do you mean SIR?

After my last post I had to go out, post some letters, go to the bank run a few chores, so Paula ran them, no point in getting changed before I had too.    Well I did (get changed) when I had to, and then after checking my e-mails I decided I fancied a beer, so I popped next door to the convenience store, where the cashier called me sir.   My first reaction was -
"What do you mean Sir?" Fortunately I realised I had got changed before I said anything, am I doing this too much?