I think it's the grass. Their very "virgin" skin is sensitive at this age. Try to keep her off the grass and wipe her off when she comes in. She may always have an issue with grass or she may acclimate to it.
Make sure you rinse extra well after bathing, especially under the belly area.
Yeah it certainly would not hurt to try a new food or protein, slow switch. I would wait until your pups belly gets better first though.
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