Ouch- Uncomfortable - Sharp stabbing shocks up the bum!?

I've experienced a sharp shocking, stab like pain right up my bum, and it sometimes hurts wen i walk or bend down, and is even worse wen i try sit down! i tried expell wind but, nothing. It is so uncomfortable. its like a trapped wind feeling but is so sharp and painfull. i rarely have this just maybe once a blue moon type thing,.. like i've trapped a nerve. Does anyone know what this might be???