Maybe one of the ingredients in the deodorant is something you are reacting to? My partner gets underarm irritation that looks like that when he uses certain deodorants, so we just had to try a few to find the ones that don't cause the issue. It's certainly possible that it's a part of a herx, but if you recently switched deodorants, then I would bet that's the cause.
Also, no body shame!
Everyone has underarm hair, and you can do anything you like with it.