Something about the sprinkle powder carpet stuff causes a contact dermatitis. I've seen several dogs would chew at their feet and belly and after ruling most everything out, they all had the common scenarios of carpet powder. Quit using the powder and the symptoms went away.

Sarcoptic is highly unlikely unless she came from a nasty kennel, and most folks I know breeding Boykins take extremely good care of them. You would likely be itching with sarcoptes as well as it is zoonotic. You can try a Benedryl (1mg/lb) to attempt to alleviate the symptoms. Sometimes it works, sometimes it doesn't.

Another thing to consider is puppy pyoderma. Young puppies immune system isn't up to snuff and they can commonly get bacterial infections on their ventrum. However, you would like notice the red pustules, and as you didn't mention that, is unlikely.