Assuming that you didn't change to a new paddle . . did you change to a new boat so that your balance/seating position changed and now your stoke is different? Are you now gripping the shaft too hard with the top hand? Did you lengthen your paddle? Change your technique?

I've got several paddles that I use interchangeably for no particular (or wise) reason. I get different pressure points or callouses at the thumb MP joints with each paddle due to the slight difference in diameter of the grip area and I suspect also from the different flex of the shafts.