I don't have personal exp- I'm measuring waaay ahead- but I AM carrying twins. However, thought of a couple things I haven't seen mentioned that you might just file in the back of your head- you could have uterine fibroids. FWIU, a lot of women already know they have fibroids when they get pg, but I imagine there are some that don't find out until well in. and they do cause you to measure big. That could explain why you feel localized movement.
Is this your first LO? If not, if you are less than 2ish years from your last pg, it could really just be that your uterus and ab aren't toned back up. I know, for me, the last of abs REALLY accentuates the uterine growth.
One thing I feel compelled to mention since you said you were looking to be informed (SO like me, btw, I want to know every. single. possibility before I go into something )- thinking on the extra fluid thing- Some women do just have more fluid- it is just that simple- and it is completely 'normal' for them and no big deal. Most cases of polyhydramnios are mild and no big deal. However, some women have extra fluid for less benign reasons- it can be because of abnormalities with the fetus, or a sign of maternal diabetes or infection. Here's a good brief overview of polyhydramnios (tho a bit heavy on the complications, in case you aren't up for going there). Seriously, I've had a quite a few friends with TONS of fluid and not a single one of them had a problem. Their babes were just good swimmers
Seems like a good idea you are getting a US, and I for one, can't wait to hear how many babes are in there! What's your mama-intuition say? One babe or two?