I'm on my second so should know this!
So I've had it in 4 years, and they deliberately cut the strings v v short. As in I could just feel the tip of them out my cervix. All fine, last week I randomly checked and they felt longer. Went for a check and they said 2cm string there 
but looked fine and am waiting for ultrasound to double check
Anyway just DTD 
and checked strings after and they're back to just out my cervix by a mm
Can it move like this?!