That's okaynormal. No worries about it. The drop in hormones causes you to have uterine contractions which results in a period. It's just the way your body normally regulates itself, except you're doing it externally.
If you start having periods very very irregularly, like, say, while you're still taking regular active bcpills, consider that it might not be working for you -- especially if y ou weigh more than 155 pounds, whether you're overweight or perfect for your body type. Women over 155 pounds have been documented to have like a 60% higher chance of getting pregnant, even when taking the pill perfectly.