Has anyone else been through this?

I went to the ER today with heavy and consisten cramping that became kind of sharp. According to the first day of my last period, and my OB, I should be roughly 6 weeks along right now. But after getting an ultrasound all that you could see was the gestational sac, which made the doctor believe I'm not as far along as was originally thought. That's fine, but then when I was discharged my paperwork said "threatened abortion/early misscarriage". I confronted the nurse, and said nobody had ever mentioned miscarriage to me, to which she replied "it's just a precaution, we need you to rest until your follow up next week."