What Is The Cause Of Missing Period?

I am 31 yp female on bc pills and my bf regularly pulls out during sex. i did not get my last period, so Thursday I took a preg test and it came back negative. Sunday I began taking my bc again, a week after i should have. i waited bc i was trying to see if my period would come. should i be concerned of the missed perios, seeing i am on bc, he pulls out, and i had a negative preg test?