I (13 years old) have blood in my underwear. Am I sick?

It’s more than likely you have had your first ‘period’.
Some families and countries actually celebrate this.
It means: you are growing up.

‘Having your period’ means:
The blood in your underwear comes from your vagina.
The vagina leads to a small cavity in your abdomen.
This cavity is called a womb.
There are two ovaries next to the womb.
There is a fallopian tube between each ovary and the womb.
Roughly once a month a tiny little egg is released from the ovary into the fallopian tube.

Now you could become pregnant.
However, in order for this to happen, you would have to have sex with a man or a teenage boy.
But you are still too young for that.
Next, the egg would be ‘fertilised’ by his semen and that means you would be pregnant.
The egg could grow in the womb.
As you are not pregnant the blood comes out of your vagina.
From now on roughly once a month the blood is rinsed out of your womb.
It takes a few days.

You can get sanitary towels or tampons for the blood in the shops or from a chemist.
You place sanitary towels in your underwear.
You push tampons into your vagina.
Ask a girl friend or your mother if they can help you.
Sometimes you may experience bleeding for a long period of time or you may have strong pains.
If this is the case you should consult your doctor.