Just a few thoughts for everyone who is extremely careful when changing their lenses and always do it in a dust free environment. As was said, unless you're in an industrial clean room, there is no such thing as a dust free environment. That said, when you put a lens on your camera, what do you do with the end cap? If you're not mating it up with, say, a body cap, you've now got something sitting out, collecting dust, either on your desk/counter, or in your bag/pocket. Do you blow it out before putting it back on the lens? To you blow the lens and body out before mounting the next lens? Unless you're that fastidious, dust will get in your camera - and even if you are, dust gets in. Once it does, even with a blower there are often stubborn pieces. Learn to clean your camera!!
That said, if you can see something on the sensor stack that looks like it could be debris on a relatively new camera, get it checked by the dealer (one reason why having a local dealer is always a good thing). Stuff goes wrong from time to time with every camera model, so be proactive when you see something - but don't jump to conclusions and expect the worst. As I said in my first post, there's nothing I see in the dust photo that shows any kind of problem similar to the D600 issue (something I'm intimately familiar with) - but that doesn't mean that there isn't an unrelated issue with your specific camera.