What Causes Delay In Periods?

Hi ,I m 34yrs old married women and have a 4yr old son. I always have a regular period of 30 days cycle. In the month of Feb ,I had unprotected intercourse with my husband. I had Ipill after 9 hrs .

My last period date was 3rd feb'11. I am still waiting for the periods. I had done pregnancy strip test on 12th March'11, it was negative.

Please guide should I retest ,wait or visit doc .......