Hello thank you for asking the question. you are getting dark blood with pain during menstruation. common cause for dark brown discharge is infection, retained old blood in uterus, etc. For pain and discharge you can use the antibiotics vaginal suppository to relieve from infection. you can maintain vaginal pH and avoid any change in pH get infection in vagina. Use the personal protection while doing sex also reduce the chance of infection and you are protected from pregnancy also. so my advice is avoid unprotected sex and take one dose of antibiotics will releive from present symptoms. hope this will helpful to you.