Brown discharge..ugh

i am on the pill for a long time. i have taken it on time for a year and more and did not miss any but mid cycle I had tan discharge and sometimes darker brown pieces. have been having this for a few months now. i contacted my doctor and she said it was common, my uterine lining is thin and sometimes can just slightly shed whenever it feels like , but I still am worried. my pills can’t suddenly become ineffective , correct? i only take my pill. not any other medication and no supplements. thanks everyone for helping.