Discovering that you have forgotten a tampon can be distressing. Losing a tampon in the vagina is not as absurd as it sounds. It really is a matter of accidentally not removing one you've put there previously.
Maybe it got to be a late day of your period, or you changed during the night, and sleepiness took over; and you simply forgot to remove the tampon. Tampons are designed to absorb a lot of moisture, although not as much as they actually used to absorb a couple of decades ago, so the moisture begins to change the flora of the vagina and causes odors. It's often not apparent for a long time that something is a bit amiss. In general the patients we find to have a retained tampon have no actual pain, and often they don't have much discharge, but complain most dramatically of odor.
Occasionally there is a significant amount of discharge, depending upon the type of tampon and long it has been there. The discharge a woman develops with a retained tampon usually is dark brown, it can even be black. The treatment could be very simple. Actually our ability to normalize the bacteria in the vagina is so great that many physicians will just treat with removing the 'lost' or 'forgotten' tampon and often antibiotics aren't necessary unless the odor and discharge doesn't resolve for a few days. Serious side effects such as Toxic Shock Syndrome even if the tampon has been there very long, are very uncommon, and if no fever or pain this is highly unlikely to be the result even from a very long retained tampon. It's important to have your gyno do a vaginal exam if you suspect a retained tampon, as it is not always possible to detect it or remove it yourself.