I promised to update here once my period returned to let you know how the cup use was going post partum, and I'm happy to report that my old Size 1 Diva Cup is working perfectly, no discomfort, no leaking, no trouble at all--it's like I never stopped! The only difference I've noted is that the cup sits lower than it used to. I used to be one of those women who had to really bear down and reach way up there to pull out my cup, but now it's just right there for me to grab. If anything, it's easier than it was pre-preg.
So to those of you who use a smaller sized cup and are pregnant or may become so in the future, I'd suggest you don't rush out and buy the larger size of your menstrual cup without first giving your old cup a try. It'll probably work just as well!