What Causes Delayed Period While HPT Is Negative?

I have delivered a girl baby on Oct 2009. Till Noce I got bleeding from Dec to till now i did not get my periods. Last month 13th I had contact with my hubby we used condoms but it teared. I have a doubt whether i might be pregnant. today did home pregnancy test its negative. I consulted with my gyne today she asked me to take meprate for 5 days 2 times a day.