Ultrasound shows pregnancy blood test shows negative?

i had my last period on May 19th i have 35 day cycles . I should have gotten my period yesterday. Had an ultrasound done Wednesday the 22nd also had a blood test done the same day to see how high my HCG level was. The ultrasound showed a sac measuring 4 wks 5 days and the ultrasound tech said looked good and was just to early to see anything else. My blood work came back saying my hcg level was less than 2. Not sure how that is possible. Could it have been to early? going back for another ultrasound on tuesday, really cant wait that long