What Causes Abnormal Vaginal Bleeding While On Birth Control Pills?

Hi I would like to know what causes my vagina to bleed. This has been happening for at least 3 weeks now. After or during a bowel movement my vagina bleeds. It feels like it s coming from my urine hole, but I m not sure. I ve been on birth control for 3 months (pill). But this ain t my first time ever on bc can u please narrow it down to what it can be. Thank you