If you aren't using Spot Metering, then the built-in flash will be in TTL-BL mode, which is effectively fill-flash. The camera attempts to balance the flash & ambient light.

I think the problem may lie with your shutter speed. The camera cannot synchronise the flash at a shutter speed faster than 1/200. If without flash your shutter speed is 1/800, then if nothing else changes the image will be 2 stops over exposed, at least, because the camera will reduce the shutter speed to 1/200 or slower. It may even be worse, as the default flash shutter speed is 1/60, almost 2 more stops slower than 1/200.

Hope this helps
Chris