What Causes Irregular Periods And Cramps Inspite Of Using Minastrin 24?

I am on Minastrin 24 fe birth control and my period did not come but all the side effects of my period came...the last couple months my period has become increasingly lighter and so I m wondering if that is why I did not have any bleeding this month. My concern is I am sexually active, but he has never ejaculated inside me. I am consistent with my pill. Once again the strange part is, I had all the cramping and moodiness but no bleeding so I am a little freaked out.