Hi there!
First thing: when something becomes very common, it's possible for us to do it automatically and not remember one particular time we did it. So, if you're just not remembering whether you took out the previous tampon or not, it's possible that you did, and your brain just didn't take a lot of notice and you don't remember.
With what you're feeling, not too long ago you asked us about feeling what was probably your cervix,
http://www.scarleteen.com/bb/viewtopic.php?f=5&t=4798 . Can you remember what that felt like? Is it similar to what you're feeling now? It doesn't have to be exactly the same, because the position of the cervix does change a little throughout the cycle. How familiar are you with how your vagina tends to feel?- if you're a bit familiar, do you have a sense of whether it's noticeably shorter than usual, or does it seem about right? If there was an old tampon there, it'd be taking up some space in the back of the vagina.
Too, when you're feeling it, don't be too rough! There aren't as many nerve endings in the tissue towards the back of the vagina, so if it
is your own body you're feeling, you don't want to accidentally hurt yourself
If you still feel very uncertain after all of this, you can always go to any doctor or nurse and have them check for you.