Random Period

Patient: I am 18 years old and I have been on the pill for close to a year now. I am pretty faithful about taking my pill each day, and at the same time. However, this month I missed a pill and took a couple at a different time than usual. I am currently in about the middle of my pack, and suddenly my period has shown up. It is heavier than spotting, but slightly lighter than my usual period. Has my period shown up because I missed a pill this month, or could there be another reason?