Is it possible to not know about a twin pregnancy?

Brooke
I am 22w5d and I have already had my anatomy scan. I went to the dr yesterday and she said I am measuring 4 weeks bigger than I should be. In the beginning my daughter told me I was having twins before I even knew I was pregnant. My test on the day of was darker than any test I have ever seen. My dr said that it is possible to have one twin hidden behind the other and since the anatomy scan showed that he is measuring the same as his due date it isn't that my due date is earlier than expected. I'm just curious if any of you have had this happen or know someone who did. I'm not saying it is twins because I honestly have no idea, just curious!