2 yo has body rash. pedi doesn't know what it is. says if doesn't go away in a month go to dermatologist? what could it be, it's all over, diaper too

Hard to say: Evaluation of a rash entails issues of location, appearance, texture or thickness of lesions, aggravating factors etc. Much of these things require a visual evaluation. Since many of the benign rashes fade on their own in a few weeks, it may do so and eliminate your need for further study. Sounds like you are on the right track.

Need pic: Viral rashes can occur but should be examined by a dermatologist.