20 week appointment, measured smaller than normal

Harley • Had my baby girl on March 9, 2017 ❤️👶🏼
Is it normal to measure a week smaller than you're supposed to be? My original due date was March 13 then it went to March 7 because baby measured big at 11 weeks. Today we went for a 20 week ultrasound at 20+3 and she said we measured 19+2 and we couldn't tell the sex 😔. Is this normal? Should I worry?