Take him to ER?

My baby has diarrhea, he’s 15 months old. He also cries when you change his pamper & says “ow”... he has no fever as well so I’m not sure what’s going on.

Before diarrhea his poops were hard-ish, sometimes he has soft poops but they’ve been hard poop balls.. could he just be constipated? Anything I can do for his soreness I’m assuming that’s on his bum?