I am 6 weeks pregnant now,but still no morning sickness and vomiting sensation.?

I went for the scanning on 5th week and gestation sac measuring 3.3mm and gestation age is 4week 1 day(8 days lesser than LMP).But now according my LMP ,its 6th week,still no sick,nausea,vomiting.Why so,?I read in one website that some times no vomiting and morning sick is the symptom of miscarriage.Is that so???and What abt gestation sac size,is that fine..?why is the difference in LMP and gestation age?