It is because you're a virgin. Just don't break it yet :)
Hello. Okay, really, I would try not to worry. It STILL could be your hymen because knowing that you broke it from a tampon probably wouldn't be accurate. You're already bleeding during your period. You could have knocked up against it and it is still intact fully or partially. Second, we also have something called a cervix. You are saying you can see this inside your vagina?
As you are 18, it's a good time to have your first pelvic exam. Women do this on a routine basis to check their health. Ya, it's kind of a weird doctor's appointment but a routine thing for doctors so nothing to be embarrassed about. This would be a good time to ask your questions regarding your anatomy.
But I would not stress over this. Sincerely. I'm sure it's normal and you are just reading too much into it. good luck