I think my tampon string came off?

Eh... its possible you did take it out and just don't remember...even if you're absolutely sure. If you put one tampon over the other, the other would have hit your cervix right away causing quite a bit of pain so you'd know...its very, veryyy rare for a string to come off, though. And, if the other tampon was inside of you... theres no place it can go... so think about it logically, its probably not in there, but if you want to calm yourself down, crouch in a squatting position in your bath tub (tub so fluids can be easily cleaned up) and sort of push, like you're peeing, and if it doesn't feel like anythings coming out, you're good.