Possibility something was missed...

Okay little background info:

I have a Boy and a Girl and their pregnancies were relatively symptomless, other than migraines with my boy.

I had an ultrasound at 5 weeks and 3 days and saw a gestational sac and a yolk sac. The ultrasound tech said singleton baby.

I'm currently 7 weeks 1 day pregnant currently. This is the most miserable pregnancy... I'm thankful... but very miserable. I'm having every symptom in the books it seems... I've been super ill with constant nausea and puking, my boobs never grew in my other pregnancies and I've already gained a cup size, I've had higher HCG levels, and so much more. I never really bloated with the other two... my second pregnancy I didn't show until I was 7 months which was surprising since he was 9lb 8oz at birth. I have what I've been calling bloat... however I've been noticing when I lay on my stomach I feel pressure of my uterus.

This pregnancy just seems ridiculously different.

Is it possible they could have missed another baby? Or am I just a loon....? I have another ultrasound next week at 8 weeks 1 day.