When parents, teachers and health workers explain the menstrual cycle to you, they will often talk in very scientific terms, or talk about it in a removed way so that it all seems to be about the egg. For example, they will explain that every month your body gets ready to have a baby, an egg is formed and moves to your uterus, but when you don't get pregnant, your body expels the unused egg and the material saved up to nourish that egg, and that is your period.
I used to say "But what about me?" when I thought about my period. Even though I knew that this egg stuff was going on inside of my body, it didn't explain my moods, my pain, or the nightmares I was having about bleeding all over the place in the middle of history class.
An uncomfortable subject
Adults hide behind scientific explanations when they are embarrassed to talk to you about menstruation. Yes, they may be almost as embarrassed as you are! And if both of you are embarrassed, it just makes it all the more embarrassing.
When your mother was a girl, her mother might not have explained much to her about what to expect. Not so long ago, menstruation was pretty much unmentionable. Napkins and tampons and douches could not be advertised on TV until the 1970's. My mother didn't even know what a period was when she started - she thought she had cancer. So even if your mother wants to make sure that you know more than she did, she might have trouble figuring out how to tell you, because no one ever explained it to her very well.
Also, menstruation is not something adults talk about among themselves very often. Women usually only talk to their close friends about their period. It is not something brought up in public, especially with men around. So menstruation is mysterious to most men, a "woman thing" that they only know about as far as effects them. This secrecy may cause a lot of awkwardness when it comes time to sit down with you and have "The Talk," no matter how determined they are to be as open and honest with you as they can.
Part of this is because menstruation is not just about bleeding, or it might be as easy to discuss as flossing. Menstruation is also signal, a red flag, that you are growing up and becoming a sexual being and that is a little weird for them. They'll get used to it, though.