Hi I understand your concern. since you have done pregnancy test and your periods are over, we should be thinking in terms of some infection.cramps followed by brown discharge has to be evaluated. possibility of pelvic inflammatory disease,meaning infection inside uterus or in the tubes and around the pelvic caviy has to be ruled out. A pelvic exam with speculam exam will rule out erosion of cervix or infection and polyps. If everything is normal an ultrasound scan can be done to see any other problems like endometriosis or fibroid etc.So you may consult a gynaecologist if symptoms persist. I hope my answer helps you. Regards