Yup, A big dust particle on the sensor.
I assume since the camera was in Continuous High that it got dislodged from the viewing screen due to the air movement created from the mirror. I always notice alot of junk in between the viewing screen as I haven't really cleaned it much lately.
I used a blower and got rid of it afterwards .... but man, that is an ugly spot.
fyi d7000, 70-300vrII