What Causes Missed Periods While Being On Placebo Pills?

Hello,I'm 28 years old.I'm on the OrthoTriCylen Lo for a couples years now.On the December 12,2010, I supposed to start taking the placebo pills for 7 days,but I started a new pack of pills to skip my period.Now,on the January 2,2011, I started taking the placebo pills,and today is the fourth day of taking them,but no period,no spooting no nothing.I already took 2 pregnancy tests:one yesterday morning and today.Both came negative!!!And I supposed to start next pack of pills this Sunday January 9 and I don't know what to do??