Could you tell me why my mirena (levonorgestrel) strings have gotten way shorter? is this bad?

Not usually bad: As the uterus moves around during exercise, intercourse or other activities, the Mirnea can move slightly inside the uterus. This can shorten the strings or even draw the strings up inside the uterus. It doesn't usually mean anything bad. If you can't feel the strings, you may want to have an u/s to make sure the IUD is still inside the uterus in good position.