Birth control and my period.

I just got my birth control and it says to wait until the Sunday my period starts. My period is sometimes irregular, especially after I have sex. I always have PROTECTED sex. We had sex the day of my expected ovulation, with a CONDOM, but the condom was starting to come down on his penis. It didn’t come off all the way, I was still protected. We fixed it. I’m nervous about precum tho. Last time we ejaculated was a week before we had sex. If my period doesn’t start today, would it be safe to just start my birth control tomorrow? I’ve read that people just start whenever. I’m supposed to get my period today, and have had no signs of pregnancy symptoms.