Still Waiting

Here I am, it is 18 dpiui and I still haven't received my period.  I had the one positive like 8 days ago, but all negative pregnancy sticks since then.  Why is my body doing this to me?  I'm ready or my period, so I can begin a new cycle.  Has anyone else had a similar thing and what causes it?  The only thing I can find online is a cyst.