Missing a period is a very common occurrence, especially when altering the way you take your birth control
pills and when changing forms of birth control. However, in order to fully assess the reason you missed your menstrual period, it is important that you make an appointment to see your gynecologist, who will be able to obtain more information and perform a physical examination.
Irregular menstrual periods are very common and can be caused by a number of factors, such as altering the way you are taking your birth control medications (as noted above), but also by other factors such as your diet and exercise, stress, or for apparently no reason at all, as hormone levels in your body can fluctuate due to a number of causes. However, in a young woman, pregnancy is always a concern when she misses her period. That will of course depend on whether you have been sexually active. And although you are on birth control, changing the way you take it can affect its effectiveness, and even if taken properly and in the same way each month, the birth control pill is not 100% effective in preventing pregnancy.
A home pregnancy test will be positive reliably only after a missed period (which it sounds like you have had). Although it is unlikely that you are pregnant, in order to put your mind at ease, it may be worth it to get a home pregnancy test. Or you could wait until your next menstrual period before taking this step. Either way, you should make an appointment to see your gynecologist, so that he or she can perform a complete evaluation.