Baby Can Barely Walk in so much pain

On Tuesday my son started having diarrhea(which he normally gets if he has anything with cows milk, bc he is allergic ) We didn’t give him anything that we would remember that contained cows milk. Today the rash is so bad that it is hindering the way he walks. He screams & turns red when I go to change his diaper. We have been using Parents Choice diapers since he was 3 months. I have been applying the parents choice diaper rash cream which normally works for him. He has had diaper rashes before from diarrhea but never this bad.. He is 19 months Any suggestions??