Have you ever taken a morning-after pill before? It very well could be screwing with your body and throwing your period out of whack. Also, the fact that you are stressed is probably affecting it, too.
I'd say that unless you always start on the same day (the 30th) every month like clockwork, you really have no reason to panic right now. And the fact that the hpts (if they were early response) were negative should calm you a little. But, if it doesn't, trust me... I understand.
If I were you, i'd give it a little more time. But if you're that worried, go get a blood test.