Canon digital?

Canon Powershot A480
Camera shake in subdued lighting in dinning room taking photo of my own oil painting with flash off to stop flash reflection on the picture. Setting shows as 1/8 f3.0 camera set to auto.
taking a picture in the conservatory setting 1/500 f 3.0 no camera shake but picture too bright.
Why do I get camera shake ? when flash turned back on no camera shake but painting reflects flash which is why, obviously, I turned off the flash.
Many thanks for reading and trouble replying.