Don't panic. That is common with DSLRs. The moment you change a lens, dust can get in there.
I wrote a detailed article about it but it is in spanish.
You basically approach this in different levels. Once you solve it you stop.
Level 1: Use the dust removal function. Not sure in the Kr but in other models the sensor can shake at startup for dust removal, or any other in camera function for that. If that doesn't work, go to the next level.
Level 2: Use a Giotto rocket blower. This is a bulb size blower you use to blow air into the sensor and hopefully remove dust. It may take several tries, but this is usually over 90% effective.
Canned compressed air is another alternative but I personally don't recommend it because it can spray propellant into the sensor. But that is a whole other debate. If that doesn't work, go to the next level.
Level 3: Dry cleaning: No, don't take it to a laundry
There are products like Lens Pen to clean the sensor. You put the camera into cleaning mode where the mirror and shutter open to expose the sensor (not really the sensor but the filter on top) and you carefully clean it. If that doesn't work, go to the next level.
Level 4: wet cleaning: Similar to dry cleaning but with special solutions designed for sensors. I've never had to go here. If that doesn't work, go to the next level.
Level 5: Professional sensor cleaning. Take it to a shop. If that doesn't work, go to the next level.
Level 6: Now you have all the right to PANIC!
As preventive maintenance, when changing lenses make sure you do it rather quickly to minimize the time exposed. Of course don't change lenses in very dusty environment like in the middle of a motocross race or a sandstorm. Try to point the camera down when changing lenses. All of this will help minimize the chances of dust.
But always keep in mind the only way to never happen is if you never change a lens, but then, what fun would that be?
Hope this helps.
Thanks,