Originally Posted by The Photo-Vet 
What causes this??? Is the flash faulty in some way? Is it the lens (Sigma EX 18-50/2.8) not communicating properly with the body (don't think so since exposure without flash or with the built-in flash is just fine).
If exposure is fine with the built-in flash then probably your Sigma flash has a problem. Check that flash compensation is set to 0 on the flash itself, maybe it is set to +1-2 and that causes the overexposure. If you zoom with your lens do you see the focal lenght display changing on the flash and do you also hear the flash head zooming? Also make sure it's really in P-TTL mode and not manual. I don't have Sigma flash, so these are just my general tips.