TMI! *baby's bottom *

My 18 day old has developed this on her bottom in the last 3 days. Now I do use pampers, but I don't think it would be from that as she is 18 days old, would she had developed this within the first few days if it was a reaction to the diapers? I have tried baby Vaseline & destin butt cream so far and it just seems to get worst & now the skin is peeling. I am changing her way more than I should and just wondering if there is anything else I could help her poor little butt.