Try less baking soda... Google natural
homemade deodorant for sensitive skin...my sis started getting a rash from too much baking soda too. She uses way less in there and is fine now and says the deodorant still works well.
To poster above ^^, there's a bunch of questionable ingredients in Deodorant (and creams, make up, bodywashes) they are not proven to harm a baby through breastfeeding but some people like to play it safe.