I think it’s been well-established in other stories on your excellent website that the act of being spanked can produce unexpected feelings in the person who is having their bottom warmed. It’s something I experienced personally during my own childhood.
I think I was about 12 when I first felt a bit ‘funny’ between my legs as my stepmother prepared me for a spanking, and by the time I was about a year older I would routinely wank after I’d been sent to bed in disgrace with a sore bottom.