The most typical reasoning for this circular obstruction is the use of a lens hood that is obstructing the flash. It could also be caused by a rather large lens itself getting in the way as well. A similar effect can be found when a wide angle lens is used that is beyond the coverage of the flash.
I would consider what lenses you were using, at what focal length, and with or without a lens hood. Adjust the combination of these things and you will resolve this issue. It is of course possible that you will have to remove the lens hood to effectively use the built in flash. Alternatively you could use an external flash mounted on the hot shoe(Yongnuo option), or off camera by using any one of a number of techniques to get the flash further from the lens(this will also improve other aspects of flash beyond resolving this issue).