I (14 year old) had my period last week. I used a tampon and I suddenly started bleeding heavily. I am afraid I may have damaged my hymen. What can I do now?

You don’t need to be afraid.
The flow of your period is different sometimes.
That may be why you bled heavily.

Do you know what a hymen looks like?
Let me explain:
Many believe it is skin grown over the vagina.
But this is wrong: the hymen is just a fold.
The fold is around the entrance to the vagina.
This fold always looks different:
Sometimes it is very small and you can hardly see it.
Sometimes it is larger and there is only a small entrance.

What’s important is: that every girl’s hymen looks different.
Only about half of all girls bleed when they have sex for the first time.
You cannot tell by looking if a girl has had sex.
So you don’t need to worry at all.
If you would like more advice you can visit a gynaecologist or a ProFamilia office (https://www.profamilia.de/)
You can find out what happens there in this question: All my girlfriends at school have already visited the gynaecologist. Do I (14) also need to go? What happens?