You should go see a gynecologist, who will be able to take a swab and look under a microscope, or do other tests. My first thought is bacterial vaginosis, since that can cause a bad smell, but it's really hard to say over the internet. Yeast infections itch, and usually come with "cottage cheese" discharge. Make sure you don't have a tampon stuck up there that you forgot to remove, because that can cause a really foul smell.