Mass of tissue at vaginal opening, not hymen, normal periods, can't use tampons it's so big and thick. What do I do? Surgery? What's this called?

Maybe cyst. Could be a large bartholins cyst. See a gynecologist for an exam to find out. Good luck!
Need exam. You need an exam to determine what is present at the opening of the vagina. That will determine what needs to be done for or about it.