It's not uncommon for people who use natural deodorant to develop a skin rash consisting of little red bumps that probably itch, burn and/or sting. I was one of those people. Chances are good that if you check the ingredient list you'll find sodium bicarbonate (baking soda) near the top. It's a shame because it really does absorb and neutralize odors, making it a wonderful - and harmless - natural ingredient for deodorant.
But, a rash is a rash and it doesn't matter how effective the ingredient is if it causes an adverse reaction.
Sometimes it's just a simple matter of application technique. People are used to rubbing their deodorants up and down, up and down without much thought to how hard they are pressing. Sometime the solution is simply to rub less and glide or dab it on gently.
If that doesn't work, you can try detoxing your pits with a clay mask made of apple cider vinegar and bentonite clay (equal parts). It seems an odd solution, I know. I can't explain why or how it works other than my homespun theory that toxins build up and live in our bodies/armpits (particularly aluminum from commercial deodorants) that are reacting with the baking soda. I say this because after applying the mask - once - I've not had a bump since. And our customers have similar stories. But even if it's not evidence of toxins interacting with baking soda and being drawn out by the clay, the mask is safe, and extremely effective, so if it works, who cares why?
You may also enjoy reading, Why Natural Does Not Always Mean Safe.