Goodluck hope you get some answers from the new GP.
My DD1 developed an awful nappy rash at about 18 months - turns out she had become allergic to the nappy wipes we were using. We had been using the same brand all her life with no problems. It looked like her bottom had been burnt, she had big blisters and red skin.
To help relieve the discomfort we had to go to just using a wet face washer to clean her. Bi-carb soda in the bath really helped as well as lots of nappy free time and lathered her in cornflour when ever she had a nappy on. It took about 6 months for it to properly clear up. She had a couple of nasty spots on her bottom that got infected and need antibiotics and a prescription cream from the dr.