I have the mirena (levonorgestrel) coil fitted and the threads seem longer, i've had intermittent stomach cramps could it have moved?

It's possible: It's difficult to tell the location of your iud solely by the length of the strings. It's possible that it has descended or is even in the process of being expelled. You should schedule an appointment with your doctor. He or she can do a pelvic examination and perform an ultrasound to confirm its location. Use a backup method of birth control just to be safe until your appointment.