It certainly could be a result of your mother. If the memory of her spankings is still fresh and they caused you discomfort, the spanking now may cause those feelings to resurface. Quite like a trigger.
I would bring it up with your T.
While most kids likely did get spanked as kids (I know I did
), your circumstances are a little different. At any rate, if it bothers you, it's worth a talk, no?