I imagine a lot of Attorneys General would jump at the chance to stand up to Amazon in defense of poor little baby bottoms and a media-friendly fact pattern like this could help bring about much-needed change at Amazon across all product categories.
The major hurdle is that you do not even know and cannot know. Maybe the product is actually legit and the visual difference comes from temperature or whatever, and the rash appeared for a different reason. One avenue may be to contact the manufacturer with the lot number.
If you try a new product and baby develops rashes or allergies, you know not to buy this product again. With Amazon you can't even conclude anything about the official product because you might have used a knockoff.
You can just conclude to stay away from Amazon for baby care products.
You have enough evidence to form a reasonable suspicion that you were sold a counterfeit item, and that it harmed your child. At this point, it would be the AG's job to figure out whether or not the product is fake, and whether or not follow up on your report.
Contact the manufacturers consumer helpline.