Why am I having sharp pain in my rectum and vagina?

Ok so I usually get this horrible crippling pain before my period or during. It starts out with me having to go poo but then when I actually arrive in the bathroom there is a horrible pain that starts in my rectum and leads to my vagina, I feel like I am about to give birth or something. What is this called?Treatments?Tips? Anything please! I really need help this pain is to excruciating and horrible for a 14 year old to have to go through. Thanks!