Is it possible that you could've had female genital mutilation? It happens in many countries and religions often it's so traumatizing to children that they pass out or don't remember, etc... Another reason could be that you have an intersex condition. I would try to see a gynecologist to figure out what's up.
It would not necessarily be visible. It's more something that can be felt, like the eraser at the end of a pencil, but that would be only during sexual excitement.