Doctors Can't figure out my due date?

So I miscarried Dec 25th on the 28th I went to confirm and at that point I had a negative blood test already (my hormone levels were so low they knew it wasn't going to last to begin with). I waited 4 weeks still no period took a test and it came out positive a couple days after that went in for blood work on Jan 29th and it confirmed I was pregnant again. Basing on that I thought I was around 7-8 weeks. I went to the doctors yesterday and they can't pin point out a date but are assuming I'm only 4-5 weeks but I've had morning sickness and all the other symptoms for weeks now. I have blood work coming up but they don't think an ultrasound would help because they think I'm so early. Anyone else run into this problem and what was your outcome?