What Causes Brownish Vaginal Discharge Along With Severe Pelvic Pain?

I have chronic pelvic pain that the doctors don't know the cause of so they have me continuously on birth control so i won't ovulate, they suspect endometriosis despite the fact that they didn't find anything during the laparoscopy. I am using the patch, orthoevra and have not missed a day or anything yet I have dark brown/bloody discharge that smells metallic.