Expecting twins! Is anyone else thinking this?

Melissa
OK so I am nearly 18 weeks pregnant and I look and feel like I've swallowed a bowling ball! I am so uncomfortable! At our 12week scan, we were told we are expecting twins and I can't help wondering, what if there's more in there? I mean what if we go back for the 20 week scan and they tell us they've found another one and it's triplets! Has anyone else had this thought or know of anyone it's actually happened to?!