When's AF due? I'm assuming you're not charting...? Because then you would know when ovulation occurred. You could have been testing too early and getting false negatives. Implantation usually occurs 6-10 days after conception, but could even take twelve days. So yes, it is possible to have implantation bleeding that close to when you expect AF. But it could be your mind playing some serious tricks with your body, if you are really hoping to get pregnant quickly... I hope it's the former and not the latter and you get your BFP!
Also, dollar store pregnancy tests! Or you can get them cheap on amazon, too.
Stumbled upon this website, shows your likelihood of false negative by day past ovulation, kinda neat: http://www.countdowntopregnancy.com/pregnancy-test/results-by-day-past-ovulation.php