Hi, I'm on the pill and sexually active, but my period was a day late this month and now it is like a dark brown! I have been stressed this past while! Have some lower back pain sometimes! Do you think I could be pregnant? Thanks

Pink, orange, red, dark red, browns of all shades - all normal.
Brown just means exposure to air, old.
Are you pregnant? Probably not. Less than 1/10 of 1% chance, if that.