I don't kno if this is just for the blood tests, but I've heard that the HCG has to be a certain level before you are actually considered pregnant. SO some tests might not show a positive because the levels are too low. I agree with satya, probably best to wait a few days, see if AF shows, and if not try again

My cycles change a bit with stress, so you could be stressing about TTC, which may throw your cycles out of whack.