What Causes Irregular And Light Menstruation With Discharge And Abdominal Pain?

I have been off birth control for over a year now as treatment for mild endometriosis.However for the past 2 mths my menstrual cycle has changed (every 25 - 26 days) before it was every 28- 30 days. Along with this change i only bleed for 2 days and on the 3rd day there is a dark brown discharge. Also, after my period i have been experiencing pain in my lower abdomen, which at times is painful to touch. I need some help. Thanks in advance.