Biology is not completely regimented. Many things can affect the menstrual flow including travel. Now that you have started Yasmin
birth control pills you may find that your cycle is much lighter. You may experience spotting or breakthrough bleeding
especially if you skip pills. It will take 3 to 6 months to see what your pattern will be on the pill. You'll find your menses more predictable on the pill than naturally because a set amount of hormones is taken every day. Biology is a little less precise. It does not sound like there's anything seriously wrong. I hope you enjoyed your vacation.