the belly scales are supposed to be rough if you feel from the tail to the head. the purpose of this is from traction while slithering. certain ways they flex their muscles and use them to help them in their forward motion.
as long as the scales are not wrinkled looking or discolored its probally fine. too much humidity could also cause scales to feel rough if you run your fingers from head to tail. if you could take pics of the belly we could tell you for sure if there is something of concern or its just normal.