Actually, if you look a little bit back, you'll see a series of pics of one of my girls building up.
Yours definitely looks like she's building, but there will start to be a noticeablely huge increase when the time really comes. It goes from being "Hmmm, I THINK she might be building..." to "HOLY COW Did you secretly eat a rabbit or something?!"
It's extremely distinct, and if you just check her once or twice a day you'll see when it starts happening.
Good luck to you!
-Jen