Can you take a pregnancy test early?

My last period was February 21st and it stopped after unprotected sex.  My period is due today but I cramped yesterday on my right side but no bleeding. I only cramp on my right side all the time now. Today I got thick eggwhite discharge instead. I had an ultrasound this morning but the lady told me to wait because she couldn't see a sack or anything yet. How long should I wait? I'm hoping my period will come on