My 1 ur old baby butt rushes

Hi my baby is 1 yr old i change her milk to aptamil 3 which diff color to stage 2... The last stage of milk she poop normal but when i start feeding her the aptamil 3 she now having diarrhea and poop 3 or 4 time a day.. her butt now is swelling (rushes) that every time i wipe her poop she cry in pain.. What should i put or cream to use for her to heal fast her butt