just starting birth control

I went to my gynecologist today because of extremely irregular periods and she started me on low dosage birth control. she said to take it the sunday after my next period ends. reading the instructions in the package, they say to take it the day my period starts. I am obviously going to listen to my gyno but I am confused. i’m not currently sexually active but if I decide to be later on, will this pill protect against pregnancy even though I started on the sunday after my period ends for the first month that I started it ?