Nicole, we're left to guess a bit because there's no EXIF.
I'll guessing you were shooting in Av mode and used a popup flash for fill. This is a common cause of overexposure as using the popup for fill will force your shutter speed to no faster than 1/180. As you were in Av mode the camera could not stop down to mitigate over exposure, the only tool left for the camera was ISO, and you were probably already at 200. So the resulting exposure will be massively blown out as the shutter speed needed was probably 3 or 4 stops faster than 1/180.
To avoid this you need a flash capable of High Speed Sync and use it for fill, never use your popup for fill outdoors on a bright day (especially in Av mode). HSS will allow for fill whilst not limiting your shutter speed to 1/180, in fact it will let it go all the way to 1/6000. Of couse though the flash power decreases greatly as soon as you are above 1/180, and gets progresively weaker the faster the shutter speed. As a result a powerful flash is needed, but even then you probably need to be within ~4 meters to be within your effective flash range. The nice thing about the Metz 48 which I noticed you've ordered is that your flash range is always displayed on the flash LCD as the flash knows your camera exposure settings and does the appropriate calculation (this assumes you are pointing your flash straight ahead as is appropriate with fill, soon as you tilt it the flash has no idea how far light has to travel to reach your subject so doesn't display a range).
Hope that helps, if my guess on your shooting mode is wrong please let us know.
cheers.
Last edited by twitch; 03-01-2011 at 05:17 PM.