I think its because your body isn't used to the smell, and after that I'm not sure... Try googling it for a more legit answer
Considering I can't get near nor change my cat's litterbox due to the smell, if I do try to change my cat's litterbox or get near it I throw up.
Yet almost everyone else in my family can get near and/or change a cat's litterbox without throwing up.
Sorry if I wasn't much help!