Had my last period end on October 14th then had sex october 17th. The 18th I took plan b which was a sunday. I had symptoms almost immediately I was bleeding as if I had gotten my period. Then that following wednesday i was prescribed medicine for strep throat. Can that cancel the affects of the plan b pill? I have yet to get my period and it is November 14th. Im stressing out alot which can also be a factor why my periods late. someone please respond.