What Causes A Missed Period After Being On Estrostep Fe For Long?

I am 26 and married. I have been taking the pill (Estrostep Fe) for about 8 years. This month I completely missed my period. For me that is NOT normal. I have taken a pregnancy test and it was negative. What could be the cause for my missed period? I have not been stressed about anything (with the exception of my missed period), I have not lost/gained any weight, I am not on any medication, and I have not changed an of my normal daily activities. Is it possible I'm pregnant even though the test was negative?