Battery life can fluctuate greatly from day to day. It all depends on the type of photography you are doing. For example, on one day, you spent the day taking pictures outside in the sunlight, with no flash, and didn't use your camera's screen to review any of the pictures. The next day, you spent the day inside, taking lots of pictures using the flash, zooming in and out on people's faces and reviewing the pictures using the camera's screen.
Features like the flash, zoom, LCD screen and image stabilization (IS) use a great deal more battery life than if you were to not use any of those features. This could explain why it suddenly seems like the batteries aren't lasting more than a day.
However, if your photography habits have not changed enough to potentially affect battery life, there could be an issue with either the image stabilization hardware or the standby mode of the camera. If the image stabilization (a fully mechanical feature) is malfunctioning, it could be activating when it shouldn't be, such as when the camera is on but you aren't about to take a picture.
I would try testing the camera by taking pictures using different camera settings.
- Turn off the flash and image stabilization, use the viewfinder instead of the LCD live view mode, do not zoom in or out. Take as many pictures as you can before the batteries die.
- Keep the flash and image stabilization on, use the LCD live view mode, use the zoom frequently. Take as many pictures as possible before the batteries die.
With test #1, you should see much better battery life than with test #2. If you still experience decreased battery life either way, there may be a hardware issue with the camera. Should this be the case, contact Canon Support
to find out whether or not your manufacturer's warranty covers this problem.