ELI5: What happens to the blood in a uterus when someone has an IUD and doesn’t get a period?

Just got an IUD. Bled the first day a little and should have started my regular period that day but did not and I’m wondering where the blood is cause I can feel the cramps. Then makes me wonder what about the years and years the IUD is in, and you don’t bleed. Where does the blood go??