ELI5: When a woman is pregnant, how does the egg suddenly get fertilized when the baby already started growing inside of her

I just don’t get it. If there’s already a baby inside of a woman, then that means she’s already pregnant. Therefore, the egg must’ve already been fertilized, right?. But I thought a baby can only grow in the womb if it’s fertilized by a sperm? So how does one fertilize an egg that’s already inside the womb, and already growing into a baby? What’s going on here? What am I missing? If you can ELI5 this for me, I’d appreciate it!