It's not a true period as such, it's a withdrawal bleed caused by the Plan B. Your period can be different, heavier, lighter, early or late. It's NORMAL.
It doesn't matter if you've taken an emergency contraceptive in the past. What happens can vary depending upon a number of factors.
You took it in time, it is unlikely you are pregnant.
Again, your next period may NOT be on time.
Hope it helps.