Getting your period while on the pill?

I've been taking yasmin for 3 weeks. I was told to skip the inactive pills and go straight to the next pack to avoid getting a period on my honeymoon. I did that but I still got my period with really bad cramps even though i've been taking the pill everyday. I knew you could spot but this is a full on period...worse then I've ever had. I thought this could only happen when taking the inactive pills?