Hey Jenna You might not fit into your pants cos many people stretch faster the more babies they have - like your uterus goes, here we go again, better make room. Or you might be retaining more fluid with this one, or...any other number of variables.
Since every pregnancy is different, it could be anything.

And obviously I don't really know your medical situation, but the Ob who isn't cut happy sounds good - that way if they say you need a C-sec, you can be more sure that it's necessary, as opposed to being just what the Ob prefers.

All the best with everything.