It's always hard in the waiting stage. Are your periods like clockwork, or can they change? It's possible you ovulated later this cycle for whatever reason, meaning your AF will come later, it's also possible you have a functional cyst (corpus luteum the egg came from trapped some fluid inside, which makes it take longer to disappear delaying AF and increasing your progesterone). What brand of HPT did you take? Some brands are much more sensitive than others - you can always wait a couple of days and retest with another brand. First response is generally the best (and most expensive). Hopefully you are pregnant, but the test just didn't show it!

Good luck!