Period was supposed to start yesterday or today? Feels like I will but haven't? Periods like clockwork!?

Last period was april 18th, 26 27 day cycle. I'm ttc and had the 27th and 28th od april (when I was fertile) I feel like I'm going to start but haven't yet. My period is like clockwork, always start on time. Took a hpt on wednesday, a first response one (Came up negative)