I actually just went through this this past weekend, Ive been on BC for three months and missed a period, it can happen. But since you have missed two in a row, I would highly recommend taking a pregnancy test to be sure. If its negative, then I would go on taking the pills, but if you miss three, I would call the doctor to make sure theres no problem.
I am sure it does happen often. What pill are you taking? As long as you know you are not pregnant, I would not worry.