the safest thing to do is not have sex you are fertile before during and after your period but if you are going to have sex use a condom or a contraceptive film (they sell it at wal-mart and most grocery stores). be safe and think about ways to protect yourself from pregnancy.
This might make the flow heavier if your just spotting but go ahead and have sex. If your partner is comfortable (I don't know your situation) then go for it and if it makes you feel any better wear a condom.
If you haven't started yet but you can feel it coming then it just might cause it to start.