from the icon is seems that you saved it as jpg or jpeg and it affected the colors, so the end result that you submitted had pixels that were automatically placed by the computer not you, which pretty much means it's not real pixel art (even though it was most likely an accident)

So.. basically, don't save as gif or jpg or jpeg in paint. Use bmp and then convert to gif on www.iaza.com or save as png and then the file will be smaller so you won't necessarily need to convert.. hope I explained it well