I could be wrong, but that paper looks to be very heavy/course. It could be nothing more than the paper pulling/catching/irratating the snake's belly scales. Change to paper towels and see if this continues. I think you'll find its something irratating the belly. Check anything that could be sharp or course in the enclosure too.
Also check the belly scales and see if anything looks out of place. Maybe check the spurs, maybe something has irratated them too.