Screaming During Diaper Change

My 15month old son just started screaming and kicking when i change his diapers. when i get ready to clean his boy part he blocks me and cries and screams. this just started happening i don't know wats wrong with him. i recently changed the brand of his diapers from luvs to parents choice because i was low on money. Thats all i can think of. He has no type of rash anywhere. never had a rash ever. I'm at stay at home mom and i watch him 24/7 by myself so anything else isnt an option.🙇🙅 anyone else went through this. i might just take him to the hospital. i been slowing around about it because I'm due with baby number 2 anyday but i know this is serious so i will take him asap