So the uterus gets really excited because it thinks there's going to be a baby. The uterus says, "OMG we HAVE to prepare for the baaaaaabyyyyyy! Let's decorate!" So the uterus gets everything ready - it hangs curtains, it buys a crib, it gets really fluffy pillows… the whole thing. (This is when the uterine walls are growing thicker with blood - "decorating", which is what protects the baby when it's growing.)
Then, once the uterus is all decorated and ready for the baby, the uterus shouts, "Okay baby, we're ready for you!" and the fallopian tubes release an egg. But the egg is really judgey. It egg takes its sweet time to travels down the tubes and into the uterus where it hangs out and judges the decor like a reality tv show. Most of the time, the egg's standards are unreasonably high. It looks around at what the uterus has done with the place and isn't satisfied. Meanwhile, the uterus is flitting around trying to woo the egg into staying with drinks and Hot Cheetos. But the egg is like, "this room is ugly and I hate it," and it bounces. (If the egg isn't inseminated then it dissolves while in the uterus.)
Once the uterus realizes that the egg is gone, it gets really sad at first and then it gets pissed. "That egg was an ******* and didn't deserve all of this preparation anyway!" So the uterus loses its **** and starts throwing a tantrum. It runs around tearing down the curtains from the walls, smashing the crib that it built, ripping open the fluffy pillows, and stuffing it all into a trash can and dragging it out to the curb to be picked up. (The uterine walls get thin and the blood drains off of them, through the cervix [the front door to the uterus' house] and into the vagina. And that's when people who get periods will bleed out their giney hole.)
But the uterus has a very short memory and a "Live, Laugh, Love" poster hanging inside her little uterus home so she quickly forgets and begins preparations, and the cycle starts all over again.