M.M.
If you were on the skinny side before you got pregnant, it is very possible to show early. I was showing real early with both of my kids... so early that people I worked with told me that it was impossible that I was showing already and they tried to convince me that I was just retaining water. I don't know about the twin thing though. I guess it's possible, but you won't know until you hear the baby's (or babies') heartbeat(s) for the first time.
Congratulations and good luck.