The IUD is inserted high up in the uterus. During the menstrual cycle, the IUD moves around inside the uterus and can therefore sometimes sink a little. But as long as the IUD is in the uterine cavity, you are protected.
Only in a few cases do women lose their IUD. If you feel a hard part, this means that the IUD is no longer in place and you should see your doctor or midwife.