I am on birth control, I started my period about a day late, but it is brown blood, do I need to worry?

No you are not pregnant. That happens commonly. You may have symptoms of pregnancy but your not pregnant. It just means that you have left over blood or the pills could be messing with your hormones. But if you really want to know if your pregnant then you should take a take home kit. Also I have heard that a woman can get pregnant when they are on their period. It only comes to be known after their last period. SO just check just in case.