Hi Phsycicsgirl, red is absolutely right, the only way to ever know for sure is keep doing tests til either one shows positive or until you get your period.
I just wanted to add though......... tests are very accurate these days, so accurate in fact, that most doctors dont even bother doing their own test after you have had a positive home test. When you fall pregnant you get a hormone build up in you that only pregnant women get, this can take time for enough to show up and give you a positive result. You may well be pregnant, but not have enough of this hormone yet.
Good luck and let us know how it all goes!