Went to ER

The other day Cuz I had a lot of tremendous cramping, I'm 6 weeks today actually. They did an ultrasound which I got to see and I saw the sac and the yolk a tad but I was scared to even look so I didn't stare too long. She said she could only see one at the moment. My question is, what are the possibilities of still having twins but not being able to see this at 6 weeks; as in if I had both babies in the same sac? Is that possible at all and it's just not being able to be seen? Sounds stupid I know lol but I'm very interested in this. She said it out of the blue I didn't even ask how many she saw!