What Causes Delay In Periods Inspite Of Taking Plan B?

I had unprotected intercourse with my bf during my ovulation time and took plan b 48 hrs after...on my most ovulating day. I had slight spotting the next week after intercourse...Just enough to show when wiping and then nothing. I m now 10 days late for my period and two negative pregnancy test....?? Is it just the plan b doing this?