Am I a bad parent?

I took my daughter to the doctors 4 days ago because she had a rash I was uncomfortable with. They said it was nappy rash and gave me some cream. Today, her rash it awful, blistered and swollen. I posted a picture here to ask for advise, and a lot of mothers told me that 'obviously' it was a yeast rash, and that her diet has caused this.

I feel like an awful mum. She's my first child, and I was only following the doctors advise. As for her diet, I can't say she eats unhealthily at all she eats very well.

These comments have made me feel like I'm not a good enough mother, and that I should know better.. do you guys agree?