Holy smokes go pee on a stick! You sound pregnant to me, especially since you can't find the strings.
If you are, the sooner you find out the better. I think the iud can be removed if it's early enough in the pregnancy.
I've been reading a few stories lately about women getting pregnant with the mirena. I have a mirena too, I love it, but I get a little paranoid sometimes.
I recently had a yeast infection and I've only ever had two other yeast infections in my life, both when I was early pregnant with ds1 and ds2. So I was a little worried, but I'm not pregnant.
Anyway, go test and give us an update. I'm really curious.