Eish, I haven't experienced this myself but a few of my mum friends have mentioned not being able to find the strings during self-checks, honestly it happens more often than people realize lah, sometimes the strings curl up or sit higher in the cervix so it doesn't mean anything is wrong. Best to just call your gynae to do a quick check for peace of mind, because they can confirm the IUCD is still in place with a simple ultrasound or examination, definitely don't stress sendiri!