Have pcos n totally irregular periods. pregnant 8weeks according to last period, but dating ultrasound says only 6weeks 2days. is this a bad sign?

Not necessarily.: Fetal ultrasound, especially in the 1st trimester, is notoriously inaccurate & so the measurements may be off by 2 wks because of the test, not the baby. Also, the estimation of dates is based on numbers, but not your own personal history, size, or genetics (are you a short individual? Was everyone in your family?). A good OB who knows your situation well will be able to tell if something's amiss.