pregnant before my due ovulation date

hiya, ive not long forund out that im pregnant! my last period was the 30th august 2008 and the only time conception could have taken place was the 5th of september 2008. my cycle is usually 30 days, my last period was horribly heavy and had to go up to the hospital just for safety. They did a pregnancy test then and it was negative. could i have actually been pregnant then or is it more likely that i ovulated early because of previous heavy period?!