Please can someone tell me what is happening !!!!?

hi i was ment to start my period on the 17th of this month but i still havent and now im four days late i really dont feel as thought im going to start. i really dont want to get my hopes up. i did do a test yesterday but it said negative which did knock me a bit as i really want to b pregnant. im just so confused. my periods are like clockwork normally and i have never been late. im not stressed thought as if im not i no i will b startin ivf in may but would really like it to happen b4 it gets to that. please any advice would b really great