Surprise!

Britney • Mommy to Zoey Belle born January 15th 2016 and step mommy to Ethan Apollo who is 8 years old.
So I had an ultrasound done at 6w because of some bleeding, and they only saw 1 baby, and the baby measured perfectly. 
I'm 10w today and when I went to my midwife yesterday, she said my uterus feels much larger than 10w and more like the end of 12w, so she's sending me to have another ultrasound done to look for twins. She also said it could feel larger because I'm tiny. Has anyone had this happen to them, where they missed a twin the first time around?