freya, I've never had multiples, but I'm pretty sure you are feeling your babies in there!!! Think about it-- 3 times the movement-- that would be hard to miss! I'm sure it will get stronger soon, and you'll have no doubt. As for being huge, I guess that is inevitable. ; )
Does your doc have any doubt about how far along you are? I just wondered, since I thought multiples usually measure small, and you are right on track or even ahead.
I doubt it is your imagination :) I am pregnant with only one little baby and I first felt her move during my 13th week... and I know that is REALLY early for having one. So, I am sure that with you have 3 babies in there, you are feeling triple the movement and sensations... so the chance of you feeling one of those movements is pretty good!!
In regards to your measurement... I really have no idea, but I would have to think that since you have 3 babies, you will probably be double/triple the size of a "regular" ol mom who is only carrying 1! Did you get pregnant using invitro or natural? If you had invitro done, then the dates won't be incorrect. But, if you conceived naturally... I suppose your dates could be off. But, you will still measure larger than a woman carrying 1 baby :)