Anyone with twins??

I went in for my 8 week ultrasound yesterday and after talking to my doctor about symptoms (excessive nausea/vomiting/exhaustion) he wanted to do an ultrasound to be sure there was only one baby, not two. Before the ultrasound he did an exam and asked if I was sure I could be further along10 or 12 weeks?! On the ultrasound he said there was only one baby measuring 8 weeks 3 days. The thing is, I wasn't surprised at any of this because since about 4 weeks when I get a positive result 3 days before my missed period, I kinda thought it might be twins for some reason. And I never considered the possibility of twins before this pregnancy. I just woke up one day like, OMG I think it's twins! Has this happened to anyone before and if so did they miss twins with your first scan??