Measuring further along?

I just had an ultra sound due to some really sharp cramping and they told me they saw the yolk sack and everything but not the actual baby since it's too early. However acording to my dr and my last period I'd be 5wks 1 day. Now they're saying I'm 6 weeks. Wouldn't they be able to see the fetus at 6 weeks? Kind of confusing. Just wondering if this happened to anyone else.