Re: What is this rash?
I agree, could be a heat rash. In addition to trying the powder, another thing that will help is avoiding diapers with PUL. Breathable cover options like fleece or wool will allow for more air to circulate, keeping the diaper area cooler. PUL is marketed as "breathable", but compared to fleece or wool it breathes much less and stays warmer. Another option is to not use a diaper cover at all, although that is more challenging when dealing with an infant. Baby leggings are a good option for keeping legs warm in cool weather when going w/o a diaper.