help!?

So my baby is almost 4 weeks old, she has a yeast diaper rash, she was prescribed some cream for it but it doesn't seem to be helping and looks like it's spreading, so maybe it isn't a yeast diaper rash? Help! I've tried unscented baby wash with oatmeal and put a little bit of baking soda in her water, stopped using wipes and use warm wet baby rags. I don't know what else I can do. I'm a FTM by the way.