Smelly poo: most cats have this sometimes. Try a different food.
Being sick: some cats do this as a hobby!
Otherwise could be a sign of worms, furball etc. Imho it is only cause for alarm when the cat seems overtaken by the wretching, rather than "putting his/her back into it". Most of us know the difference between a cat who's
trying to be sick and one who is really ill.
Also, the nature of the (sorry to say the word again!) sick itself is a clue to whether it's normal or something to be alarmed about; for example, feline infectious enteritis is horrid and yellow; furball has a little mouse-shape of fur in it, usually etc. Bear in mind that too much vomitting leads to dehydration. Does your pud drink after throwing up? (Normal.)
Sorry about all the
talk. Makes a change from the other end!
