The fact is that only you know precisely what was in your frame, people here make suggestions for you to solve the problem. Some may be way off, and some might actually help.
Another thing to consider is the metering mode used.
If it's set to meter the small portion of your frame and your subject is dim compared to the surroundings, in exteme cases, the surrundings may be greatly over exposed. But if all the frame is over exposed this may not be the case.
Sometimes the light, as we percieve it, may be missleading.
The meter on your camera is more objective. The question is what did it show when you took that shot. Did it show over exposure or not ?
If it didn't, maybe you have used exposure lock and reframed the picture.
That could give you that effect.
Bu if the over exposure is consistent in many frames, that couldn't be the case either.
Mabe you haven't payed attention to the exposure meter and used a shutter speed / aperture pair from another situation. I'm sure you know that those won't change when you half press the shutter button in manual mode. You use the ones you have set previously, those could be from a week ago. The first day I used the 50D I forgot that the camera is set on manual and took a few shots as if it was in program mode (using aperture and shutter speed that had no relation to the actual shot), The pics came out beautifully white
At the moment I can't think of anything else.
But this doesn't mean that this is all.
If you want a shallow DOF you can(almost) always open the aperture and use the shutter speed that gives you the correct exposure.
Hope that helps
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera