Hi I'm Supposed To Start My Period Yesterday, And Have

hi I'm supposed to start my period yesterday, and have not, I started spotting on Saturday and I thought it was my period but it was brown, now I'm a day late and still haven't started. I've been feeling really nauseous And have had diarrhea for a couple of days. Also I'm on a birth control I've never been late or spotted like that. I've taken a pregnancy test and it was negative