why does my baby cry when i change his diaper?

Today every time I changed his diaper he started crying like if he was in pain. I checked and he doesnt have a diaper rash. When I take off his diaper he doesn't cry, its when I wipe his butt that he starts to scream and cry like if he was in pain. I dont know what to do and I dont know why he's in pain. I have tried baby powder and it hasn't helped.