Was your period not due yet? If so, a test may not be accurate. (Remember, 18 days after sex isn't necessarily 18 days after conception ... sperm can live in your body for several days.)
Wait a few days and, if you still don't get your period, test again.
I've just had a pregnancy test, and the result was negative. I know that I had sex 18 days ago though, so would I get an accurate result, even if I didn't take it the day my period was due? I'm only 15 so I don't really know (and I really regret it obviously, drunken mistake)