What Causes Delayed Period And Vaginal Spotting While On Expired Implanon?

My implanon has been expired since 12/24/2014 and I've been having regular period for the last 7 months. My period normally starts around the 5th of each month. I haven't started my period for 15 days. today I started to bleed but just lite spotting, I've took a pregnancy test and it came out negative. Could i be pregnant or with the implanon being expired could it make my period be very late?