Well if your period was due on the 17th and you tested on the 20th thats not to early cuz with my second pregnancy I got a + only 9 days past ovulation. However with my first pregnancy I didn't get a + test until I was two weeks late for my period.
So I would wait until you are two wks late and then re-test. If it is negative and still no sign of period then call your doctor and they will give you a blood test to measure the hcg levels in your system then this will confirm!