What Causes Delay In Periods?

hi, my husband and i had union duirng my fertile week i.e 17th may 2011 and no protection. i didnt get my mensus which supposed to be by 30th may but at the same time over last weekend i.e 4th may i had viral fever and took some medications i.e paracetamol and strng flu medicine (no antibiotics). my pregnancy test yesterday mornign was negative. but i still havent gotten my mensus. i'm confused. pls enlighten

height 151cm, wieght 51, age 31