How to explain what a period is to my daughter

My daughter just turned 7. The other day she overheard me telling my sister I’m on my period. She asked “what’s a period?” And I said I’ll explain when you’re older. But I started thinking, she could start her period as early as 8-9 years old. My daughter is very mature for her age, I’ve already talked to her a little about sex and how it’s for two people who love each other & that’s how babies are made. I just don’t know how to explain to her what a period is in a way she will understand. How did you moms do it? I would hate to not talk to her about it soon & she did start her period next year. That would traumatize her!!

So those who are questioning why I gave her the sex talk before the period talk & those who are saying they would never talk to a 7 year old before sex, maybe you should consider there’s a specific reason why I had that talk with her. Me & my husband broke the news to her this year that he’s not her biological dad. So of course a 7 year old doesn’t understand what exactly biological and nonbiological is, so I explained in order to have a baby, two people must have sex. I explained that her biological dad is the person I had sex with and created her. I didn’t go into great detail about periods & all of that stuff at the time because I just broke some very life changing news to her, it wasn’t really the time & place to be going into anatomy & how the body works.

I came here looking for advice on how to explain periods, not to be judged because I told her about sex first.