Starting the pill

Just got my first set of pills yesterday. They’re the combo pills. I’m supposed to get my period next week. My doctor said I could start my first one tomorrow or wait until the first Sunday after I start. What can I expect if I start tomorrow as far as my period goes? Or if I start taking them while

I’m on it? She said either would be fine, just wondering about the effects. I’m already having PMS symptoms right now.