I'm asking because I used to be able to feel the string (or whatever it is) and I was on vacation a few months ago and had horrible cramps and felt like the iud was scratching me inside (if that makes sense) and now I notice that I can't feel the string thing. I'm starting to get a bit paranoid that it fell out and that I could end up pregnant again. That wouldn't be the end of the world but it would stretch out tight budget to the screaming point.
I would use a back up method of birth control and make an appt with you gyno to get it checked out. They can check with an ultrasound if they have reason to believe it fell out.