17 days no AF

Sarah
What it says on the tin.. I'm 17 days late, haven't tested in 5 days but I'm going crazy. My next AF is due around the 28th so I wonder if I'm gonna see that one. 
My GP gave me 2 hpt's so I might do them at the weekend. 
Anyone had similar experience? It's just strange I'm as regular as clockwork then as soon as I have unprotected sex my AF disappears!