Hello I Am A 17 Year Old Female And I

hello i am a 17 year old female and i have frequent sex with my boyfriend. Him, nor I have ever had any sti's or std's so that can't be the cause of this pain. Recently i have been having this weird pain in what feels like my uterus after I empty my bladder. It goes away once i stand up though. I'm not sure what to do or what could possibly be wrong with me. please help!