Good morning Nari and welcome to blogging. They may not have "stolen" it, but what happens is that if someone is following your blog (test it on another you are following) and you have written a post that they think their followers would like to read, at the bottom of the blog there are "share" symbols, google buzz, facebook, twitter etc. One of those is a "blog this" symbol. If someone selects that it will copy your post to their blog.
Don't be offended by this. It's nice to think that someone enjoyed your posting. It also means that it may bring you more followers as it credits your blog automatically as where the post came from. So you may find you get some new followers. Don't be angry but be proud someone enjoyed it.
Hope that helps.