TLDR: Your sensor's entire area isn't exposed by the shutter all at the same time, so using too fast a shutter speed with a flash results in only a fraction of the image being lit by the flash. Don't use a speed past your camera's stated flash sync speed (most modern cameras have it at 1/125 or 1/250) when you have a flash in use.
(or should it be TLDW? :P)
Baka. Soku. Zan. - The justice behind the dysORDer.