Empty gestational sac at approx 6 weeks?

I have now had two ultrasounds and both ultrasounds are showing an empty sac. My last ultrasound was last Friday. The sac was measuring 6 weeks 3 days but all that was in it was what they think is a yolk sac. I have PCOS and my periods are very very irregular. If we go by my LMP then rightly I should be about 9 weeks. They were guessing I conceived around April 19 which is no possible. I was on my period for about 20 days straight and my husband and I were not having sex. The only time I could have possibly gotten pregnant was at the beginning of May. Which would mean by right Im probably actually no more than 4 or 5 weeks pregnant. And an ultrasound wouldn't show anything this early, right? I guess what I am asking is has anyone else had a situation with ultrasounds showing an empty sac and then everything being ok? This is my 5th pregnancy.... I lost the previous 4. So this is a huge deal to me and I am walking on pins and needles. Any personal stories, experiences, anything anyone might have to help build some hope would be greatly appreciated.