When silver tarnishes, it’s a surface discolouration caused by the interaction of oxygen (air) with the silver, and it’s also known as oxidization. With sterling silver this process is normal, natural, expected, and yes, it can occur relatively quickly.
Many things can speed tarnish- air pollution, the minerals in your water (and as such wearing your silver jewellery in the shower can speed this process too,) living or working near a chemical, electric or manufacturing plant are all things that can cause discolouration more quickly. Humidity in the air hastens tarnish: other tarnishing agents include wool, felt, paper and cardboard, food (especially eggs and onions), rubber bands, latex gloves, and certain paints. The skin oils on our fingers can be added to the list. (If you are a chemist, you will recognize that a common element among these tarnish-producers is sulfur.)
Recent studies have shown tarnish develops microscopically within hours of being cleaned and exposed to air again. You may not see it on the piece for several days or weeks, but it’s there, and will show up as a black residue on a polishing cloth. A great way to keep seldom worn jewellery clean (and silverware if you have it), is to store sterling silver in a ziplock bag with all the air squeezed or rolled out of it. You can wrap it in a soft cloth first too. Oxygen and thus tarnish may still seep in and can still cause blackening especially if the pieces are stored for long periods of time, but it should keep things to a minimum.
Only soap and water, silver polish or a silver polishing cloth should be used to clean silver. Many household cleaners and chemicals can alter your precious silver.