How old are you? If you are still in early puberty, your cycles can be very irregular.
Your periods can be delayed for many things, stress, change in nutrition, lack of sleep, strenuous exercises, low body fat, health problems, loss of weight and others. Your body is not a machine that is always the same. Your cycles can change from time to time by itself.
If your vagina is otherwise healthy (no infections), I would not worry too much about it unless your periods stays away for a long time. It just happens some times.