Yes, it happened to me. You don't really know anything until an u/s. The only theory that I have heard to explain it is something along the lines of they believe that most pregnancies start off as twins, but one is absorbed by your body to ensure that you have a strong viable baby. But since most women don't get u/s done early in the pregnancy, they don't know for certain how many pregnancies actually do start off as twins, and it is too cost prohibitive to do a real indepth study of it, to do an u/s with all the pregnancies in a large enough area.
Anyway, my last one was definitely twins at first. I had u/s at around 8 or 10 weeks, because we weren't sure how far along I was. By the 4 month, there was only one baby. I had no pain, no spotting, or anything. I believe that Discovery Health has a documentary about it, I don't know if you can find it, that might help.
at 6:30 PM on May. 16, 2010