Silver tends to tarnish due to the metals it is alloyed with copper. Someone maybe had instructed you to clean your sterling jewelry with toothpaste, it’s really wrong, as one kind of cleanser toothpaste it is abrasive and it will dull the finish over time with tiny scratches. You’d best use silver polish, and a no phosphate type detergent is also OK, or simply storing them in anti-tarnish bags or anti-tarnish jewelry boxes.