Please help question about being pregnant with twins?

hi all well this is my story had a m/c in march and had a d/c on the 12/3/08 i concieved again shortly after i also went for a scan and because the baby was to small to see how far along i was they measured the babies sack which was 6wks and 5 days (must have concieved literally few days after d/c) thankfully all is going well and i am 9 weeks now however my concern is when i had the scan there was only one baby visable but im showing rather a lot for 9 weeks and also feel so sick all the time my midwife made a joke asking if i was pregnant with twins however there was only one on the scan is it possibl still to be pregnant with twin has anyone every experienced a early scan when only one baby was seen however when you went for a 12 week scan it was showing two babies. i realise this is a odd question to ask however i am showing at only 9 weeks . thankyou for any comments please no slandering me as this is a very sensitive matter after what happened in march.