I have also done this . I realized it the other day and figured it had been in for two or three days I also had a very bad smell but it was from the old.blood in the tampon I had forgotten about. Thankfully when putting the new tampon in it was very difficult and simply didn't feel like it was in right. I used a shower head just to rinse the inside of my vagina to make sure any old blood and or discharge was able to come out. I never had and sick like symptoms but this is also the second time it has happen to me. I am goinn to make sure from here on out that I am more careful when inserting tampons. I am also off of my period and it has been a couple days since it has happened. I feel fine should I still make an appointment?
It is very serious to leave a tampon on for too long. It can cause TSS. Toxic Shock Syndrome. Every tampon brand has information about it in the box or you can read about it online. Here is one website.
http://www.mckinley.illinois.edu/handouts/toxic_shock_syndrome.html
I wouldn't worry about it unless you have or begin to get other symptoms. Just be aware of your body and make sure you don't notice any signs of infection. Though if you don't have any signs/symptoms now, you probably won't get any and you're fine. Don't try to wash out your vagina, douche, or anything like that because that in itself can cause an infection. As long as you don't notice any problems, you'll be fine and don't need to do anything. Just make sure to be aware in the future to make sure you change out your tampon regularly. Take care.