well... I could be mistaken but from what I recal from sex ed... if you are WAY inside those could be your tubes back to your ovaries... however I'm not sure how big they are.. I mean if you can actually feel them or not. Anyway your cervix separates your vagina from your uterous(sp). It's what keeps a baby from flying out any old time it wants
JK Anyway sometimes during sex your partner can "bottom out" and actually hit or open your cervix. This can be both painful or pleasureable.. usually painful. However for the guy it feels like he has found another vagina cause it rubs his head the same way your lips do when he enters you. Also the cervix is where the diaphram rests if you ever use those for birth control. Anyway If I were you I wouldn't go exploring that far back... I mean there aren't really any GOOD nerve endings back there and you don't want to spread infection to your eggs.