My son has allergies that can cause rashes in the groin area. Have you had her tested for allergies? It could be a contact allergy, such as soap, or the elastic in her underwear, or it could be a food allergy, or an animal allergy, etc. My son is so allergic to cats, that he breaks out if he so much as sits in a chair, where someone who owns a cat has sat. When he was younger, he broke out if he had eaten anything with egg in it (though he has since grown out of that allergy). It always starts in the groin area because it is a very sensitive area, I've been told.
If she's had this for weeks, and the suggestions of your doctor have done nothing, she needs to be looked at by a doctor. If your doctor looks at the rash, he may determine that it looks more like eczema than diaper rash, and recommend allergy testing, or determine that it looks like something else entirely (yeast perhaps?) and follow a different route. It is difficult to diagnose over the phone.