Why is my hymen different?

So inside my vagina I understand that I have the hymen membrane, but for me it is shaped like a complete ring on the front side of my vaginal wall. Behind that ring is the "rest" of my vagina and that is where i insert tampons. I don't understand what this ring is because it is very small and I dont know if i should try to open or stretch it out. I'm very confused because I've never had sex before but I've been fingered and the last time that happened my partner tried to stick their finger in the small hole inside my vagina instead of the larger one. I'd really like to know if there is something wrong because it hurt a lot when they did it and I dont understand what the small ring is.