I Have Started Taking Birth Control Because I Have A

i have started taking birth control because i have a very irregular period, i got my period 2 times a month and it was a very heavy flow. My doctor prescribed me birth control and told me to take it the first day of my period although when i did get my period i forgot to take them and took them 2 days later which landed on a tuesday. I then got off my period on wednesday but have been using the pill regularly everyday at 10pm and am about to go into my 4th week. I am not sure because i did some research about sunday starters or day one starters and how some people miss their period and i am not gonna lie i had sex on thursday but did use a condom. Im not sure i am just very confused about the pill and how it works and will like an explanation. I apologize for the long letter. Please get back to me if you can. Thank you.