What Causes Missing Periods And Negative Pregnancy Tests?

i missed my periods. it was supposed to come on 7th june. i had had protected sex with my husband and also taken ipill within 24hrs of the act, i did a pregnancy urine test on 12th june using mankind preganews which gave me a clear "negative" result. I have still not got my periods. please let me know what can be done ahead? are there chances of preganews being wrong? am i pregnant inspite of having protected sex and ipill??