What can I do if I had a tampon stuck inside me for 2 days and now there is a odor?

As with any other time, if you have a bad vaginal odour it's a sign of a problem such as a vaginal infection. Tampons are a major cause of vaginal infection, so if you accidentally left a tampon for two days then this increases your risk of infection. Chances are you have Bacterial Vaginosis, thus you need to go to your doctor for tests and treatment: they will give you antibiotics.