You'll actually get your next period a couple days after starting the placebo week of pills. Your body responds to the withdrawal of the hormonal pills by shedding the lining--which is your period.
Yes, eventhough you take the placebo pills on one Sunday, your period may not even start until a couple of days after that. For instance, I was on the pill from age 14 - 24 (10 years) and I would take the placebo pill on a Sunday, period would start Tuesday night or Wednesday afternoon, and I would continue to take the placebo until the following Sunday in which I would take the active pill and eventhough I was taking the active pill everyday after that, my period wouldn't stop until the following Tuesday or Wednesday. So you see, it is a 28 day cycle because you take the pill one day and it takes your body a couple of days to react and start/stop your period. I hope this answers your question, just keep doing what you're doing, you're doing it right.
your body will adapt to your birth control pills. Just take as directed and try not to miss any and you will be just fine.