I need help finding out what I'm hitting when i finger my girlfriend. Very curious!?

When I finger my girlfriend, i usually just like rub against something with my fingers when im inside here, idk if its like a wall of her or something because im very illiterate with that, as you should abviously know. it sort of feels like an organ, or just something that you would find on the inside of someones body. can anyone tell me what im hitting? by the way, it gives her plenty of pleasure.